Reaction
Typically used with Courage.
In short: get ready to rumble. Reaction represents how on top of things your character is, and how ready to take immediate action. When a potentially hazardous situation arises, a Reaction roll is used to determine in what order each participant can act. For instance, Reaction checks are called for at the outset of any combat. The higher the roll, the sooner the character can act. The character with the highest Reaction check result gets to act first, the character with the second highest acts second, and so on down the line.
Reaction is also used to see if a character can react quick enough to avoid some sort of danger. For instance, if a trap is sprung and the characters are pelted with poisoned darts, the GM might ask for a Reaction check at a given DL. The characters who fail the check are caught wide-eyed by the trap and are at a penalty to avoid its dangers. We'll give you more on how Reaction works at the beginning of the Combat chapter.
Opposed Rolls
Reaction rolls are compared with other Reaction rolls to determine order of action.
Retry
Reaction attempts cannot be retried without the expenditure of a fate point.