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Full Idea
'fld R' indicates the 'field' of a relation, that is, the set of all objects that are members of ordered pairs on either side of the relation.
Gist of Idea
'fld R' indicates the 'field' of all objects in the relation
Source
Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0)
Book Ref
Enderton,Herbert B.: 'A Mathematical Introduction to Logic' [Academic Press 2001], p.4