I'm very interested in this question because the first book I purchased after D-Day was "Surviving Infidelity" by Rona B. Subotnik and Gloria G. Harris, which includes exit affairs:

Quote
Some people use the affair to get into therapy so that the therapist can make a difficult leave-taking easier by helping the betrayed spouse cope. Others use the affair to make the spouse angry enough to do the leave taking that the unfaithful one is afraid of initiating. And still others have an exit affair to have someone to lean on during the leave-taking.


As much as I wanted my WW to read the book, I didn't want her to read about exit affairs, because I didn't want her to say, 'oh, yeah, that's the affair I had' and promptly exit.

As has been noted, most Waywards claim that they wanted to get out of the marriage a long time ago or were unhappy for years, so the fog babble of a WS does not get to define what is or isn't an exit affair. I think an A can only be called an exit affair after the WS has truly exited. Never beforehand and never by the WS.

BH (Me): 33, XWW: 33
Married 1999, No kids
EA: 11/04?-10/07, PA: 05/07
D-Day: 06/07
Divorced: 04/09
Affair is over for OP but not for WS
WW wants to move away w/o me
WW moved away w/o me