example: currently you have 3 defence.
This means you can have, in total, 9 ranks worth of techniques spread across your defensive moves. For example, three rank 3 moves. Or a rank 3 move, and three rank 2 moves.
note that the individual rank of a technique can not be higher than the stat you use for it - so you can currently not have rank 4 defensive moves.
Once you have a position however, you can have higher rank moves that fit that position.
right now, you have, for example, 2 rank 2 techniques for shinsoo defence. Since your shinsoo defence is 3, you can however have 9 ranks worth of techniques. So you are 5 ranks short.
if you choose to leave those five ranks empty, you can fill them later, using exp. The only advantage you draw from that is that you do not have to spend exp to increase your stats, which would subsequently increase your repertoir. Those right now who have 9 of 9 points spent can not learn any new techniques, unless they spend 2 exp to upgrade their repertoir size of a certain stat, or outright increase that stat. It's a bit of a tactical decision. Leaving spots open now means you are quicker on learning new ones.
Minor spoiler: in-game events and in-game character progression may lead to learning about rarer, special techniques that may or may not be better than what you can learn right at char creation. Also, you might simply want to learn techniques that provide a way of dealing with a rival.