SEPM generates the delta definitions on the basis of stored defs revisions in the database - so the delta here is differential. For that to work SEPM needs two defs revision stored - one initial and then the latest target def => delta then being simply content that is existing in the latest defs but not in the initial one - as you mentioned the "difference between the two".
Example:
* SEPM definitions:
- Definition Revision 1
- Definition Revision 2
- Definition Revision 3
-> possible deltas:
- Delta A - difference between Revision 1 and 2
- Delta B - difference between Revision 2 and 3
- Delta C - difference between Revision 1 and 3 - this one will be bigger than A and B because it covers all the differences between 3 revisions
= differential
LUA on the other hand doesn't really create any deltas on its own - these are passed down the line from the internet liveupdate symantec servers and stored on LUA. These are larger than SEPM deltas because they are incremental and not differential - they contain all the "differences = new signatures" between the initial Hub definition and the target current definition.
Example:
* Definitions from Symantec/LUA:
- AV HUB/Full Definition
- Definition 1
- Definition 2
-> possible deltas:
- Delta 1 - difference between HUB and 1
- Delta 2 - difference between HUB and 2 etc.
= incremental
The main difference here is then that SEPM kind of takes into consideration what revision of the definition is already on the client and creates the delta to it. LUA is not concerned about that - provides simply the latest revision that will be appropriate for the client.
I hope I didn't make any errors in the above description and examples.