While choosing targets as part of casting a spell or activating an ability, your opponents must choose at least one Flagbearer on the battlefield if able.
6/8/2016 Any player’s Flagbearer may be targeted. For example, if each player controls a Coalition Honor Guard, Seal of Strength’s ability may target any of them.
6/8/2016 Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keywords are activated abilities and will have colons in their reminder text. Triggered abilities (written with “when,” “whenever,” or “at”) don’t have to target a Flagbearer.
6/8/2016 If a spell or ability’s targets are changed, or if a copy of a spell or ability is put onto the stack and has new targets chosen, it doesn’t have to target a Flagbearer.