There are three elements in the linkage between a User Account and a Profile page:

A user account may have an externalAuthId

<http://vivo.mydomain.edu/individual/u8041>
a <http://vitro.mannlib.cornell.edu/ns/vitro/authorization#UserAccount> ;
<http://vitro.mannlib.cornell.edu/ns/vitro/authorization#emailAddress>
"jeb228@cornell.edu"^^xsd:string ;
<http://vitro.mannlib.cornell.edu/ns/vitro/authorization#externalAuthId>
"jeb228"^^xsd:string ;

runtime.properties may contain a value for selfEditing.idMatchingProperty

The profile page may match the externalAuthId on the user account