Most maps have 2 event tiles, identified by sparkles or dots on the tile. If a unit stands on one of these tiles, they may get more experience, weapon proficiency, or an item. At what point (map start, tile activation) is the type (and exact item) of the event tiles generated?
The effect of the tile is randomly chosen when one of your units ends their turn on it.
This effect is randomly chosen based on the unit that activates it, so it needs to be done at time of activation. For example, if paired units activate it those units can have their relationship improved as an effect.