Here it is possible to see what each general’s abilities do and how strong they can be when given 1 to 3 points. (click on URL of end this article).
To be able to assign it to a general at all, you must assign at least one point to the abilities in the Skill tree.
The moment you have activated an ability in the skill tree, you can move to Abilities and choose the abilities you need for your general.
The abilities are in groups.
I will explain with the example you see above. Here the abilities Intimidate, Dragonscale Armour and Lasting Wounds are in the same group (for one slot). You can only use one of these abilities for this slot. I only activated Dragonscale Armour in the skill tree and ignored the other abilities. If you wanted to activate Lasting Wounds, you would assign the points to that ability and ignore the others. It would be a waste of points that you can assign to offensive/defensive skills, for example.
Here is a little help where you can see how each general’s abilities are organized in groups per slot
Horatio
Toril
Leo the Hunter
Alyssa
Sasaki
All generals is on this URL: there.