Acronym
Acronyms surrounded by square brackets are constructed from the author initials and the year of publication. For example, the acronym [AB70] comes from Arp+Bertola 1970ApL. Acronyms without square brackets do not refer to the reference code, but rather to the catalog name. For example, XMMM31 is the acronym for XMM-Newton - Messier 31 field.