What is the "TBA policy"?

Is it written anywhere?

If you are referring to the T&PA (Transition and Process Agreement), here is what it says:

Pilots who, at the time of implementation of an integrated seniority list, are
in the process of completing or who have completed qualification training
for a new position (e.g., B-777 Captain or A-319 First Officer) may be
assigned to the position for which they are being or have been trained,
regardless of their relative standing on the Integrated Seniority List.