I think that the Baptism Nullification letter is letter to have one's JW baptism annulled/revoked and perhaps to have it considered invalid. But I might be wrong about that. I am starting to look into was is said in some topic threads about, such as at https://www.jehovahs-witness.com/topic/47666/baptism-nullification-update and at https://www.jehovahs-witness.com/topic/46515/extrication-via-baptism-nullification . I notice that there is an update at https://www.jehovahs-witness.com/topic/47981/baptism-nullification-endgame . A post in it says the following.
"Well the GoverningBodyGod has added a new twist to my BN letter. They
are not contesting my stand that I was never a Baptized member. But
they have decided to treat me as if I was an unbaptized publisher. This
put me in the approved associate standing.
After all, I did
associate and had many privileges so I can't claim otherwise. They
simply announced that I was not associated with the Christian Cong.
anymore."
I am not sure it would work for those who while an adult had filled out and submitted an application to be a regular pioneer and became a regular pioneer. I am also not sure it would work for those who became a ministerial servant or submitted an application to work at Bethel.