In Yggdra Union, in order to use the Card Skill of a Tactic Card, a unit that meets its Ace requirement must be present. The unit whose weapon affinity matches the ace requirement of the card must lead the Union in order for the Skill to be usable; it is displayed on top of the card in battle, and at the bottom when in battle preparation. Please note that some skills will have other criteria to meet; some skills are exclusive to a specific character, and others require that the target is in a certain type of terrain. Enemies are exempt from the Ace requirement rule, but still adhere to the skill's specific needs.
The Ability: Always Ace allows the unit to ace any card regardless of weapon affinity.