"a" is short for address I assume. A way to refer to events that are not addressable by hash/id because they don't have a stable identifier since they can change. Contact list, metadata, replaceable and parameterized replaceable events fall into this category.
Discussion
yeah, it seems to be some sort of makeshift to deal with inadequate broadcasting of such public events
and i think that it might be mostly neglected and/or abandoned
i'll update with a note about what i learned from the search code