The idea of the <code>ETag</code> is that one can edit/update resources without overwriting the changes made by somebody else.
The <code>ETag</code> is therefore some kind of state representation which can be checked on the server when a change is requested from the client to see whether the state on the server is still the same as the client assumes it to be.
Since the REST API may not be the only client to the LDAP server and the REST API can't protect itself from concurrent writes to the LDAP on the same objects (due to the PHP principles of request-based execution), the LDAP must therefore most likely be part of the solution for generating and validating ETags.