Experience in Destiny counts towards 3 separate areas:
- Your level, up to level 20
- Your active skill subclass
- Your equipped items
If you kill a mob, for each 1 experience point you get, 1 experience point is attributed towards each of the 3 separate areas above.
Let's take an example.
You kill a Dreg at level 15. Dregs give 20 experience.
- Your level gets 20 more experience points towards level 16
- Your active subclass gains 20 more experience points towards the next skill(s) to unlock
- Your equipped armor items get 20 experience towards their next upgrade points (if they can be upgraded)
- Your equipped weapons get 20 experience towards their next upgrade points (again if they can be upgraded)
If you kill the same Dreg at level 20, your level does not gain any more experience and will stay at 20. You need armor with +Light stat to gain more "light level" above 20. However, the rest of the areas get the same experience as before.
This means that you can unlock all skills on both subclasses with time.
Note You do not unlock skills because you level. Since both your level and your skills get XP, to begin with it seems that every time you level up you gain a skill but that is just because the same amount of XP is needed both for the next skill and the next level. If you momentarily switch to your other subclass and earn some XP, the level and skill XP will be out of sync, so you'll gain a level when killing one mob, and gain a new skill a bit later.
Things I don't know:
- Do all weapons get experience, or only the one(s) you used to kill the mob with? If you used two weapons, do they both get XP, do they get 20 each or a portion of the 20 each?
- Do all armor items get 20 each, or a portion of the 20 (say 5 each)?
- Do all skills that currently have a progress bar get 20 each, or a portion of the 20 (say 10 each if you have 2 that currently have a progress bar)?