A message drawn on a slant is interpreted as an event if the target is a reactive class, and as a primitive operation if the target is a nonreactive class. A slanted message emphasizes that time passes between the sending and receiving of the message. Slanted messages can cross each other.

A message drawn as a horizontal line is interpreted as a triggered operation, if the target is a reactive class, and a primitive operation if the target is a nonreactive class. The horizontal line indicates that operations are synchronous.
