The Leiden Conventions are since 1935 the most common rules, symbols, and brackets used to indicate the condition of the original text in an epigraphical (or papyrological) edition.
[...] a lacuna or gap in the original text, not restored by the editor (extent known, each dot = space for 1 letter)
[— — —] a lacuna or gap in the original text, not restored by the editor (extent unknown)
[abc] letters missing from the original text due to lacuna, restored by the editor
a(bc) abbreviation in the text, expanded by the editor
<ab> characters erroneously omitted by the ancient scribe, restored or corrected by the editor
{ab} letters in the text considered erroneous and superfluous by the editor
ạḅ characters damaged or otherwise unclear in the text, ambiguous outside of their context
... traces of letters on the surface, insufficient for restoration by the editor (Greek and Papyrological
usage)
ABC clear but incomprehensible letters
[[abc]] letters deleted in antiquity (rasura)
v. or vac. space left empty (vacat) on the stone or page
\ a / Letter inserted by scribe
abc Letters that were visible to previous editors, but are no longer visible, and are accepted by the current editor
In editions of Latin texts you may found the following recent conventions:
+++ traces of letters on the surface, insufficient for restoration by the editor
⌐a b ¬ Letters or syllables corrected by the editor
NB Some (mainly French) authors follow Louis Robert in his preference for an older system, which he explains in: L. Robert and J. Robert Fouilles d'Amyzon en Carie, Paris: De Boccard, 1983, pp. 9-11 'Signes critiques du corpus et édition'.