I just don't know which lentils are chosen for which types of recipes and why?

Any recipe (mine or someone else's) should indicate which lentil you need to use. For example, in the red lentil dal, you would use red lentils. In the Sweet Potato Dal (EHH) the recipe also calls for red lentils. In the Yellow Dal (HHC) you use yellow lentils (also called yellow split peas). In most cases, you cannot substitute in a different lentil than called for because the taste and texture are so different.