/usr/share/dovecot/dovecot-abi contains that portion of DOVECOT_ABI_VERSION
from configure.ac outside parentheses (i.e not including the package version.)
It might look something like this:

2.2.ABIv10

...packages providing plugins for dovecot can use this information in their
dependencies to avoid having to update every time there is a minor update to 
the dovecot package.

