AFF: abbreviation for "Away for Flow", which indicates that the person is being in the zone, thus, shouldn't be disturbed (hence the "away").
In this era of remote working where a coworker does not necessarily know the physical state of another coworker, this abbreviation can be used to politely hang/reject a
conversation (or chat) when the person is being focused (i.e "AFF, will reply later").
About flow: "flow, also known colloquially as being in the zone, is the
mental state of operation in which a person performing an activity is fully immersed in a feeling of energized focus, full
involvement, and enjoyment in the process of the activity." (Wikipedia, at en.wikipedia.org/wiki/Flow_(psychology))