P, D, L (Place, Dimension, Linear), and the other kinds.
Usually, you'll place dimensions on the Drill Drawing, or a mechanical layer. Personally, I prefer to put the mechanical outline (with dimensions, holes, board outline and any significant features) on Mechanical 2.
You should read up on geometric dimensioning, and drafting, too, if you don't know how they work. It's easy to miss dimensions or put in ridiculous tolerances if you aren't thinking about it.
Tim