Sometimes, the tooltip will say, "Learning all 9 ranks of this skill will allow you to learn: "RANDOM SKILL""
For instance, learning all 9 ranks of Heavy Escort Captain will allow you learn Attack Pattern Beta III.
Read it again. You don't learn it. You are actually able to teach it. To any Bridge Officer of that discipline. If I'm understanding correctly, with that, someone can trade you a (given your example) Tactical BOff, and you can teach that skill to that BOff and trade it back.
While on Spacedock 1 (Earth Spacedock), you can train another one of your officers in a skill that you are able tot each (by getting 9 points into certain skills). You don't use it yourself, it teaches one of your BOFs how to use it.