In Eplan you have the option of creating different sets of identifiers for different standards, countries, and customers. Identifiers can consist of multiple characters, for example KT or KTP. The identifier sets are stored project-specifically as schemes.
In the project settings (Devices > Numbering (online)) you specify which set of identifiers you want to use for a project (for example DIN). Every function definition is then assigned a certain identifier. When creating a function the corresponding identifier is transferred from the function definition to the device tag. This can be manually changed. Identifiers are always entered in the function and are not referenced.
See also
Your Own Connection Point Designations and Identifiers