The second sentence tell us that there are 2 categories of Gods:
Benevolent Gods - Wealth, Officer, Seal and Food.
Not benevolent Gods - Devil, Hurting, Robbing and Blade.
The benevolent Gods should be used in the 'follow' way while the not-benevolent Gods should be used in the 'against' way.
The third sentence says when the useful gods in the chart are correctly used, they result in noble structures including the not-benevolent gods.
So now it comes to the question:
What is the 'follow' way and what is the 'against' way?
The 'follow' way is explained as:
是以善而顺用之，
则财喜食神以相生，生官以护财；
官喜透财以相生，生印以护官；
印喜官煞以相生，劫财以护印；
食喜身旺以相生，生财以护食。
When it is benevolent and used in a 'follow' way, is like
Wealth likes being engendered by Food God, and Wealth engenders Officer to protect wealth; Officer likes protruding Wealth to engender (Officer) and Officer engenders Seal to protect Officer;
Seal likes being engendered by Officer and Devil to engender Seal and Rob wealth to protect Seal;
Food likes prosperous self to engender Food, and engendering Wealth to protect Food.
The 'against' way is explained below:
不善而逆用之，
则七煞喜食神以制伏，忌财印以资扶；
伤官喜佩印以制伏，生财以化伤；
阳刃喜官煞以制伏，忌官煞之俱无；
月劫喜透官以制伏，利用财而透食以化劫。
Not-benevolent and used in an 'against' way is like
7th Devil likes to be controlled by Food, dreads supports and help from Wealth and Seal; Hurting Officer likes to be controlled by Seal, engendering wealth and transforming itself,
Yang Ren likes to be controlled by Officer and Devil, dreads the total absence of Officer and Devil,
Robbing Month likes to be controlled by penetrating Officer, using Wealth and penetrating Food to transform Rob.
此顺逆之大路也。
These are the major ways of the concepts of 'follow' and 'against'.
So ZPZQ says the Useful God must be considered from the month's branch, what say other books?
SMTH does not have a particular chapter that discusses Useful God, but in one of its chapters, it is mentioned that Useful God is mostly taken from the month's branch 况用神多取诸月 in Volume 2 - Discussing Year, Month, Day and Hour 论年月日时
SFTK also mentions that Useful God comes from the Month's Decree, there's a poem that teaches how to derive the Useful God from the month Stem in sFTK
As for DTS, the concept of 'Usefulness' is much more complicated.
DTS Chapter 13 writes:
道有体用，不可以一端论也，要在扶之抑之得其宜。
Tao has forms and uses, cannot be discussed from one direction, from support and repress that benefits would be obtained.
体 - Body, form, structure.
用 - Use, capability, ability.
This is a very profound sentence, the explanation is given by the original annotation below:
有以日主为体，提纲为用。
There's one that uses DM as the form, the outline (month's branch) as the use.
日主旺，则提纲之食神财官皆为我用；日主弱，则提纲有物，帮身以制其神者，亦皆为我用。
DM prosperous, the Eating God, Wealth and Officer in the outline all is used by me; DM is weak, the object in the outline that support self and control other gods also can be used by me.
提纲为体，喜神为用者，日主不能用乎提纲矣。提纲食伤财官太旺，则取年月时上印比为喜神；提纲印比太旺，则取年月时上食伤财官为喜神而用之。
Outline as the form, Favorable God as use. DM cannot use the outline. Food, Hurting, Wealth and Officer are too prosperous, thus Seals and Compares from Year, Month and Hour are taken as Favorable God; In the outline when Seal and Compares are too prosperous, thus take Food, Hurt, Wealth and Officer from Year, Month and hour as Favorable Gods to be used.
此二者，乃体用之正法也。
These 2 are the correct method in determining 'form' and 'use'
有以四柱为体，暗神为用者，必四柱俱无可用，方取暗冲暗合之神。
It is also taken the four pillars as 'form' and obscured gods as 'use', must be in the four pillars nothing can be used, thus is it taken obscurely clash or obscurely combine Gods (to be used).
有以四柱为体，化神为用者，四柱有合神，即以四柱为体，而以化合之神可用者为用。
It is also taken four pillars as 'form', transformation god as 'use', there's a combination god in the four pillars, thus is taken four pillars as 'form' and the transformation god as 'use'
有以化神为体，四柱为用，化之真者，即以化神为体，以四柱中与化神相生相克者，取以为用。
It is also taken the transformation as 'form' and four pillars as 'use', when the transformation is real, the transformation god is regarded as 'form' and four pillars together with the gods that engenders and counters as 'use'
有以四柱为体，岁运为用，有以喜神为体，辅喜神之神为用，所喜之神，不能自用，以为体用辅喜之神。It is also taken the four pillars as 'form', years and luck as use, also with favorable god as 'form' and the gods that assists the favorable god as 'use', the god that is favorable, when cannot be used be self, thus it uses the god that assists the favorable god.
有以格象为体，日主为用者，须八格气象，及暗神，化神，忌神，客神，皆成一体段。
There's also structure as the 'form', DM as the 'use', it is when the Qi image, obscured god, transformation god, dreadful god, guest god all become on body.
若是一面格象，与日主无干者，或伤克日主太过，或帮扶日主太过，中间要寻体用分辨处，又无形迹，只得用日主自去引生喜神，别求一个活路为用矣。
If it is a one sided structure or image, nothing is related to DM, or when DM is overly injured or countered, or when DM is overly assisted, and in between we look for 'form' and 'use', and it cannot be found, there's no alternative but to use DM itself to lead and engenders favorable god, to seek out an alternative path of living as use.
有以日主为用，有用过于体者。
There's also DM as use, and use is more than form.
如用食财，而财官食神尽行隐伏，及太发露浮泛者，虽美亦过度矣。
Such as using Food and Wealth, and Wealth, Officer and Eating God are all completely hidden, or when overly revealed, superficial or floating, even though is beautiful but it is excessive.
有用立而体行者，有体立而用行者，正体用之理也。
There's stillness as use and movement as form, and there's movement as use and stillness as form, it is truly the principle of 'form' and 'use'
如用神不行于流行之地，且又行助体之运则不妙。
Such as when the useful god does not move at the prevalent places, and it moves to the luck which supports the form, that is not auspicious.
有体用各立者，体用皆旺，不分胜负，行运又无轻重上下，则各立。
There's form and use that establish (separately) on their own, both of them are prosperous, equally, and when in the luck cycle they are also equal, they would be separately establish own their own.
有体用俱滞者，如木火俱旺，不遇金土则俱滞，不可一端定也。
There's form and use that are entirely sluggish, like fire and wood are equally prosperous, does not mee with metal and earth so they are entirely sluggish, this cannot be determined from one aspect of the matter.
然体用之用，与用神之用有分别，若以体用之用为用神固不可，舍此以别求用神又不可，只要斟酌体用真了，于此取紧要为用神，而二三四五处用神者，的非妙造，须抑扬其重轻，毋使有余不足。
Use in the concept of 'form' and 'use' is different from the concept of 'use' in useful god, it is now allowable to take the 'use' from the concept of 'form' and 'use' as Yong Shen, and if to ignore this and to look for alternative useful god is also not allowable, as long as the true 'form' and 'use' is thoroughly thought, and from that the important useful god is determined, and 2, 3, 4, 5 places with useful gods, is not good chart, has to modulate its heaviness and lightness, do not use the excessive or insufficient.
The explanation, unfortunately is also very profound (to me), anyway, from bits and pieces that I could understand, it seems like the first two sentences from this excerpt agree to the concept of 'Useful God' in ZPZQ.
有以日主为体，提纲为用。
There's one that uses DM as the form, the outline (month's branch) as the use.
This sentence is essentially the traditional ZiPing method. The objects in the month's branch can be taken as the Useful God, depending on which HS protruding and has the most influence in the chart.
日主旺，则提纲之食神财官皆为我用；日主弱，则提纲有物，帮身以制其神者，亦皆为我用。
DM prosperous, the Eating God, Wealth and Officer in the outline all is used by me; DM is weak, the object in the outline that support self and control other gods also can be used by me.
This describes the concept of 相神 Ministerial God that is taught by ZPZQ. Let's leave this for future discussion.
Other application of 'form' and 'use' concept from DTS have their own uses, and helps to determine the 'Useful God' for miscellaneous structures 杂格.
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