The payload: namelist/<param> values as an object, or the <content> value.
The event name (event / eventexpr).
OptionalsendidThe send's id (id, or generated for idlocation), if it has one.
The target (target / targetexpr); what it means is up to the processor.
The processor's canonical type URI.
A message produced by
<send>for a non-SCXML Event I/O Processor.