The list below summarizes the treatment of this message element in this tutorial through Step 3.4:
In Step 2.4 the message v_CheckItemAvailability is automatically created during flow recording and is empty (see Table 1 of Step 3.2: Add message elements to the message files).
In Step 3.2 you add the message element available to the message v_CheckItemAvailability (see the subtopic Variable message in Step 3.2). By default the initial value of a new String variable v_CheckItemAvailability.available is an empty string.
In Step 3.3 you add a mapping to the mapping routine of the Assign node named Assign that sets the value of v_CheckItemAvailability.available to the string YES if the requested item number is found (see Add two mappings to the mapping routine of the new Assign node in Step 3.3).
In Step 3.4 you created a mapping that copied the contents of v_CheckItemAvailability.available into the output variable o_CheckItemAvailability.available (see Store a return value into the output message).