As a card action or free card action, a unit with the Observe X keyword can choose an enemy unit at 
and in LOS. The chosen enemy unit gains X observation tokens. Observation tokens are removed during the Remove Tokens step of the End Phase.During an attack, a friendly attacking unit can spend any number of observation tokens that belong to the defending unit during the Reroll Attack Dice step. The attacking unit rerolls one attack die for each observation token spent. Observation tokens are spent one at a time, and the same die can be rerolled multiple times by spending subsequent observation tokens or aim tokens. The attacking unit may spend aim tokens and observation tokens in any order.