Full Idea
In the 'compound' notion of sum, the mereological sum is spread out only in space, not also in time. For it to exist at a time, all of its components must exist at the time.
Gist of Idea
An 'compound' sum is not spread in time, and only exists when all the components exists
Source
Kit Fine (Things and Their Parts [1999], §1)
Book Reference
-: 'Midwest Studs in Philosophy' [-], p.63
A Reaction
It is hard to think of anything to which this applies, apart from for a classical mereologist. Named parts perhaps, like Tom, Dick and Harry. Most things preserve sum identity despite replacement of parts by identical components.