The best thing is to not mess with any of this crap unless you need it. If you don’t need it and want to use it anyway (not that HCG will do you any good), at least understand it first. That means lots of reading and learning, and you won’t find the info on a PE forum.
In TRT, use HCG to keep testicle size up and for the other reasons set out by the doctor mentioned in the article above. Basically, it gives more complete replacement than only testosterone. The article I posted is by one of the most knowledgeable TRT doctors in practice today. He is neck to neck with Dr. Shippen. Unfortunately, many doctors, even endocrinologists, are woefully ignorant about male hormones and the protocols for TRT.
In AAS use, particularly with extended cycles, Dr. Crisler recommends low dose HCG all through the cycle. Makes sense to me. You’d want to stop it somewhere around the end of the cycle, the exact timing depending on what you were using.