Square Brackets
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, [ ] may also be used to mark deletions." (Maas 1958:22)
A rule of thumb is that, in papyrological and epigraphical texts, [ ] is supplement, and in codices, it is indeterminate between supplement and delete.
In Philodemus Phil., $*PERI\ POIHMA/TWN &(lib. i) (P. Herc. 444, 460, 466, 1073, 1074a, 1081a) (1595 002), [] indicates 0 or 1 missing letters, while [ ] indicates a lacuna of indeterminate length.
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.