Hi quills, Ken, and everyone,
It's a smart idea to behave like the Useful God element. Previously, I only thought of engaging in activities related to the UG. I remember I had this same question about the different types of Useful God a while ago. Some calculators like the Joey Yap's or the Chinese one output 3 elements for the Regulating Useful God. But then, I suppose there is only one element that might best serve as the Useful God.
Found this somewhere in my notes, perhaps only a small part of what Ken meant by many categories of Useful God. I'm really not good at translation, it reads a bit funny

. Hope it can still be of some use here. Feel free to correct.
Three categories of Useful God:
1) 生扶用神 Sheng Fu Yong Shen就是看日干强弱,得力不得力情况。观察四柱是否有克身,泄气之神煞,或者帮身太旺,再看四柱有没有伏制之吉神,如能伏制化煞,并能生扶命宫,入贵格,是为可用之神,命理学称为生扶用神。
This method looks at the strength of Day Master (weak or strong) + the most dominant 10 Gods. It observes whether there are plenty of elements in 4 pillars that attack the DM, or help strengthen the DM. Then also check whether there exist elements in the 4P that restraint the good and bad elements, as well as capable of supporting the life palace. Those helpful elements are considered the inborn supporting Useful God (Sheng Fu Yong Shen).
Example applications:
1) 日干弱, 多官杀, 用神取印星。
Daymaster is weak with a lot of Officers (DO & 7K): use resources as Useful God.
2) 日干弱, 多财星, 用神首取比劫
Daymaster is weak, many wealth stars: Useful God is companions (Friends & RW).
3) 日弱多食伤, 首先要取印星为用神.
DM is weak with many outputs (EG & H0). Firstly, have to use resources as Useful God.
4) 日干强旺, 多印星, 取财为用神.
DM is strong with many resources: wealth stars become Useful God.
2) 调候用神 Diao Hou Yong Shen - This might be the Regulating Useful God (usually there's no specific element, more than one element ??)
This is a way to adjust when the chart is too hot or too dry, e.g., a hot chart likes moist and vice versa. Some calculators provide these Regulating Useful God, which makes us question the difference between Regulating UG and Useful God only.
For example,
癸日干生 - Gui DM 亥月 born in Hai month.
庚、辛,水多用戊,金多取丁。
Too much metal & therefore too much water, use Wu earth to control water. Use Ding fire to control metal.
丁日干生 - Ding DM 寅月 born in Yin month
用甲必须兼用庚金。
If using Jia wood, have to double up the use of Geng metal (so that it can chop Jia to produce fire).
3) 通关用神 Tong Guan Yong ShenLook at which elements counter which, then find the remedy to clear the conflict.
在相对平衡的四柱中, 财星与印星旺而相战, 有一点官杀星可通关; 印枭与食伤旺而相战, 日干得令可通关; 官杀与比劫旺而相战,有一点印星可通关。
In a relatively balanced Bazi chart:
- Wealth stars counter the resource stars. A small amount of Direct Officer or 7 Killings can clear the conflict.
- Strong resources (direct/indirect) meet prosperous Eating God and Hurting Officer will counter each other. The season of the DM can clear it (depends on what season the DM was born).
- When officers (DO and 7K) meet prosperous Companions (Friends & Rob Wealth), they counter each other. Use resources to clear it.
Source:
http://www.zhouyisoft.com/bazi/7.htm,
http://www.xjw.cn/8z3.htm