attributes tag

["jehan.procaccia" <jehan.procaccia@int-evry.fr>]

Hello,

I see in lot of docs /sample ldif ... the use of a tag to an attribute, ei:
description;lang-us : directory
description;lang-fr : annuaire
or the famous "binary" tag for userCertificate for exemple .

Is the use of these tags defined/normalized somewhere ? I've (rapidly) 
looked at RFC but didn't found it :-(
what I want to do is to link a multiple title to a different 
departementNumber so I would have something like this:
title;dept-compter-science : teacher
title;dept-managment : director

can I create personnalized tags ? or are they normalized (we often see 
lang-xx or binary but what else ... ?)
Thanks.