If Caitlin exhausts to use her ability, does she target upon activating the ability or upon resolution? (If she targets an enemy unit at a battlefield, and then a reaction causes it to no longer be there, does she have to target herself or a different friendly unit if there are no enemy units at battlefields?)

Ruling: Caitlyn targets on activation, not on resolution. The vast majority of targets are chosen on activation, only a small few are on resolution.

Nuances:

• If the original target becomes invalid before resolution (e.g., removed by a reaction), the ability would need to retarget according to its targeting requirements at the time of activation