To this I agree wholly. The effects themselves are not really clear in what they do. But really, that's pretty much the same for every game. If you want to really understand how it works, you need to wait for players to break the mechanics and make guides. I did my part of number crunching in the past and still do, but I'm not much of a guide writer

Unless you are just talking about training the abilities you unlock via maxing certain skills, this isn't correct.

I have trained all my BOs how I want them. I'm Tactical and I can train my Science and Engineering officers via the npc on Earth Dock.

If I'm missing a detail do let me know.

Certain skills let you train your boffs with new abilities. For example, training the captaining skill Canon Weapons to 9 lets you train boffs Canon: Rapid Fire III. But you can only train that ability if you're a tactical captain. So an engineering captain with canon weapons at 9, wouldn't be able to train that new ability to a boff.

The Skill Trainer in Earth's Spacedock only trains Skill Levels I & II, he has no III's to train.

Yes, the game epically fails to mention the distinction and the skills window essentially lies to you.