In sequence diagrams, Modeling automatically renders the execution specification of a message that shows the period of time when the message is active. When you draw a message link to the destination lifeline, the execution specification bar is created automatically. You can extend or reduce the period of time of a message by vertically dragging the top or bottom line of the execution specification as required.  

It is also possible to create an execution specification on a lifeline without creating an incoming message link. In this case a found message is created, that is a message that comes from an object that is not shown in diagram. Use the Object Inspector to hide or show the found messages. 

Messages in sequence diagrams have their origin in an invocation specification. This is an area within an execution specification. The notion of an invocation specification is introduced in Modeling's implementation of UML 2.0 sequence diagrams. Though this element is not defined in the UML 2.0 specification, it is a useful tool for modeling synchronous invocations with the reply messages. In particular, invocation specification marks a place where the reply messages (even if they are invisible) enter the execution context of a lifeline, and where sub-messages may reenter the lifeline.  

Active and passive areas of the execution specification are rendered in different colors. The white execution specification bars denote active areas where you can create message links. The gray bars are passive and are not a valid source or target for the message links.