Full Idea
{a1, a2, ...an} indicates that the set comprises of just those objects.
Clarification
[the numbers are actually written as subscripts]
Gist of Idea
{a1, a2, ...an} indicates that a set comprising just those objects
Source
Graham Priest (Intro to Non-Classical Logic (1st ed) [2001], 0.1.2)
Book Reference
Priest,Graham: 'Introduction to Non-Classical Logic' [CUP 2001], p.-7