pkla-admin-identities is an internal helper program of pkla-polkit-compat. You shouldn't need to run it directly, except for debugging purposes.
Configuration is read from files with a .conf extension in the /etc/polkit-1/localauthority.conf.d directory. All files are read in lexicographical order (using the C locale), meaning that later files can override earlier ones. The file 50-localauthority.conf contains the settings provided by the OS vendor. Users and 3rd party packages can drop configuration files with a priority higher than 60 to change the defaults. The configuration file format is simple. Each configuration file is a key file (also commonly known as a ini file) with a single group called [Configuration]. Only a single key, AdminIdentities is read. The value of this key is a semi-colon separated list of identities that can be used when administrator authentication is required. Users are specified by prefixing the user name with unix-user:, groups of users are specified by prefixing with unix-group:, and netgroups of users are specified with unix-netgroup:. See the section called "EXAMPLE" for an example of a configuration file.
pkla-admin-identities outputs the resulting configuration of administrator identities, one identity per line, using the same format (including e.g. the unix-user: prefix). If no administrator identities are configured in the above-described configuration files, the output will be empty.
The following .conf file
specifies that any user in the staff UNIX group can be used for authentication when administrator authentication is needed. This file would typically be installed in the /etc/polkit-1/localauthority.conf.d directory and given the name 60-desktop-policy.conf to ensure that it is evaluated after the 50-localauthority.conf file shipped with pkla-polkit-compat. If the local administrator wants to override this (suppose 60-desktop-policy.conf was shipped as part of the OS) he can simply create a file 99-my-admin-configuration.conf with the following content