About Time in Grade (TIG) requirements

So I was just helping a friend of mine with some career advice (Idk why he asked me lol, maybe because I'm a new supervisor or something) and a good question came up for us regarding Time in Grade requirements and things like that. Now, the way I understand it, is that since I started my career as a WG employee, and moved to the GS system, in order to apply to higher GS positions I need to meet…