A globally unique id that identifies the sender of a message.

By default, an AEP Engine uses the hash code of the engine name as the sender id.   

A monotonically increasing sequence number.  

A sequence number of <=0 indicates that the message is not sequenced i.e. receivers should not perform duplicate checking on it.
A sequence number of 1 indicates that the start (or restart of a stream) i.e. a receiver that receives sequence numbers 1,2,3,1,2,3 should not consider the 4th message a duplicate.
Otherwise, receivers should consider a sequence number not greater than the previous sequence number as a duplicate 

Note: The AEP Engine uses the same sequence number space across all the channels on which outbound messages are sent. This means that receivers that only express interest in a subset of channels on which a sender engine publishes messages will not receive consecutive sequence numbers. Therefore, gaps in received sequence numbers cannot be used to determine message loss. The X runtime ensures zero loss via the use of Guaranteed (at-least-once) quality of delivery on the underlying transport coupled with persistence to deliver exactly once delivery to application message handlers.