Package: *
Pin: origin my.custom.repo.url
Pin-Priority: P

Package: *
Pin: origin packages.sury.org
Pin-Priority: 999

P > 1000
    causes a version to be installed even if this constitutes a downgrade of the package

990 < P <=1000
    causes a version to be installed even if it does not come from the target release, unless the installed version is more recent

500 < P <=990
    causes a version to be installed unless there is a version available belonging to the target release or the installed version is more recent

100 < P <=500
    causes a version to be installed unless there is a version available belonging to some other distribution or the installed version is more recent

0 < P <=100
    causes a version to be installed only if there is no installed version of the package

P < 0
    prevents the version from being installed