That's a general concept, I guess you could say the rules cover it:
For the other training types, players will improve more quickly not only with additional playing time, but also a more skilled player will improve more quickly. For example, a great inside defender and rebounder will find it easier to improve his shot blocking than a poor inside defender and poor rebounder would. This encourages you to develop players with a combination of skills, but there are many combinations of skills which work well together -- you can develop players who will be a good fit for your team's needs.
Yes, that is pretty vague, but the impression I get is that all outside skills relate to each other for training purposes, and the same for inside skills.
Charles has a very busy weekend in front of him, otherwise I'd ask him to comment here.
NO ONE at this table ordered a rum & Coke
Charles: Penn has some good people
A CT? Really?
Any two will do
Any three for me
Any four will score
Any five are live