MEP

What is MEP Plan? What is the different types of MEP? What is Controlled Group and Affiliated Service Group? What is the difference of CG and MEP? What is unrelated and related Employer in MEP? Is it necessary that MEP plan should be Controlled Group? Is Combined Testing is mandatory in MEP? When is MEP is preform combined testing?

Is there any IRS documents which explains all these things? Or any other documents.

Multiple Employer Plan - a type of plan that covers 2 or more unrelated employers

Types or MEP? are you asking the diffrence between a PEP and MEP? not sure what you are asking here.

CG and ASG - see IRC §414(b), (c) and (m)

CG is related employers, MEP is generally unrelated but may have members of a CG adopt.

Unrelated and related generally defined by common ownership or not, though ASG can create related without ownership interests in some cases.

Getting into all the testing is beyond the scope of answering.

Yes there are lots of documents starting with the Internal Revenue Code and Regulations. If you are in an employer capacity I'd suggest seeking of a TPA who works with MEPs and CGs that help you, if you are in the field I suggest taking to you employer about resources to learn.