I'm not sure this is the right place for this question, but here goes.

What is the meaning of average annual compensation? Does it include salary, overtime and other benefits? Is it just yearly salary or any combination?

Thanks in advance

It can be any or all. It depends on the language of the plan document. Most plans use W-2 compensation, but certainly not all. The Summary Plan Description should have a definition, if you can't get a copy of the plan document.