Experience Points (XP) are almost always granted for a specific [Skill] or other ability of some kind. In this case, the points are added to the existing point total (or create a point total in that [Skill]) and the bonus granted is computed from the new point total.

In the rare case of XP being awarded without a specific reference to what they should be spent on, they should be spent on some skill that is frequently used on screen in a significant fashion.