Refer to the TLG Beta Code Manual for a full listing.
Primarily these brackets are used to provide editorial/conjectural additions in the case of damaged text.
However, "Where the manuscripts have not suffered physical damage,
A rule of thumb is that, in papyrological and epigraphical texts,
In Philodemus Phil., $*PERI\ POIHMA/TWN &(lib. i) (P. Herc. 444, 460, 466, 1073, 1074a, 1081a) (1595 002),
In the CANON OF GREEK AUTHORS AND WORKS, brackets around a name question the authenticity of that name, or even the existence of the author.