Full Idea
The 'general sum' of all objects satisfying a certain predicate is denoted by a variable-binding operator, expressed by 'σx(Fx)', read as 'the sum of objects satisfying F'.
Gist of Idea
General sum: the sum of objects satisfying some predicate, written σx(Fx)
Source
Peter Simons (Parts [1987], 1.1.08)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.15
A Reaction
This, it seems, is introduced to restrict some infinite classes which aspire to be sums.