In reference to this, I thought I'd try to help out. Rules sections are in blue, my comments in black. The Rules for Card Features say this. An ability is a block of text that creates an action. Abilities are written in a mixture of boldface and normal text, and include one or more of the boldface designators “Limited:” “Open:” “Battle:” or “Reaction:” that indicate at what point the action can be taken. (The boldface text “Focus Effect:” is not an action designator but a special trigger for a trait.)
The Rules for Actions and Abilities - Designators say this. A player may use an ability to take an action at the timing point indicated by its designator: Limited: At an opportunity to act in the player’s Action Phase; Open: At an opportunity to act in any player’s Action Phase; Battle: At an opportunity to act in the Combat Segment; Reaction: At its stated trigger.
The Rules for Battles say this. The Combat Segment is an action round. The Defender gets the first opportunity either to take a Battle action or pass. That opportunity then goes to the player to his left, and follows the sequence of play leftwards after that. After all players pass in sequence, the Combat Segment ends and the battle proceeds to the Resolution Segment. Emphasis added. The only thing that uses up the opportunity in an action round sequence is an action of the appropriate designation, per the rules for Abilities. For Battles, that is abilities with the Battle designation (i.e. Battle actions)
The Rules for Timing say this. Limited, Open, and Battle actions each have a time they are taken in the sequence of play for the Action Phase or Combat Segment. Emphasis added. The indicated action designators use up opportunities in the appropriate action round. Reactions and triggered traits or effects (described in detail in this section do not. While this is not formally stated anywhere it can be inferred from the rules about Ability Designators, and the fact that the timing rules are specifically in relation to reactions and triggered traits and effects.
About the only thing that I see that is missing and might make things cleaner is that there is not a formal definition for Action Round, except under the Action Phase rules. If there was an entry for Action Round in the glossary section the other rules could refer to it instead of players having to know to look for the definition in the Action Phase section since none of the other sections directly refer to the Action Phase section. I am not sure it is a common enough question to warrant a FAQ, but it might not be a bad idea.