Answer by fkraiem for Install package from a specific distribution with private apt repository?

Your Release files should contain a Codename header to allow pinning by release name. From man apt_preferences:

The Release file is normally found in the directory .../dists/dist-name: forexample, .../dists/stable/Release, or .../dists/wheezy/Release. It consists of asingle multi-line record which applies to all of the packages in the directorytree below its parent. Unlike the Packages file, nearly all of the lines in aRelease file are relevant for setting APT priorities:[...]the Codename: line    names the codename to which all the packages in the directory tree belong.    For example, the line "Codename: jessie" specifies that all of the packages    in the directory tree below the parent of the Release file belong to a    version named jessie. Specifying this value in the APT preferences file would    require the line:        Pin: release n=jessie

See for reference the Release file of the official Xenial repository.