Q also uses [a] to denote markers, like in "[Obama] [9]" recently.
Also something like "[:22]" or "[:13]" has been used, likely to denote timestamps (e.g. minutes).
But I wouldn't exclude other possible meanings as well, since square brackets also had been used as in example like "[F]", "[-30]", "[14:40]", "[2019]", "[1-4]" and others.