We have two issues:

• KEY-123
• KEY-456
  And 3 JQL/conditions:
• Condition A is matched by KEY-123 and KEY-456
• Condition B is matched by KEY-123

Scheduled execution:

When condition A is met:
-- If condition B is met: add label B
-- Else, add label C

To many programmers, the expected behavior would be:

• KEY-123 gets labeled as B
• KEY-456 gets labeled as C

Only KEY-123 gets labeled as B and KEY-456 isn't edited at all.
On a next execution, if no issues are met by condition B, then KEY-456 gets labelled as C.

Note: The path of IF/ELSE conditions aren't executed for each issue individually, but for any match in the rule's trigger condition.

If the rule's trigger brings 20 issues and a IF matches 2 and ELSE, 18, the rule won't perform the actions of the IF block for the 2 issues and ELSE's block for 18, but only the IF's for the 2 issues.

The ELSE block only gets executed if none of the issues matches the IF condition.