The main purpose of this document is to make roster not only a "contact list", but also a "list of useful items". This means that the user has the ability to store not only users' JIDs, but any JID that he wants to quickly access with more information than just the name, subscription and roster groups.
All information, that can be stored for each item can be divided into three categories:
jabber:iq:private as in Bookmark Storage (XEP-0048)  for storing this information has one big problem: it is hard to mantain roster data in two separate places. When a client is online, then the client application can handle
jabber:iq:roster changes and make similar changes in private storage, but when the user is online with a few different resources, or when he is offline, then making the information consistent is very hard task (a roster can be changed when user offline, e.g. if someone is making an account transfer).
But we have a place where this problem does not exist:
jabber:iq:roster. We can store it in
<item/> subtags. Existing server implementation MUST NOT remove
<x/> tags from it. In this case all information always relates to its JID and disappears when this JID removed.
JID, category and type are stored as attributes of
<item/> tag. Categories and types are the same as in disco. Official categories and types associated with disco are administered by the XMPP Registrar  see <https://xmpp.org/registrar/disco-categories.html>.
This information is implementation-dependent, so to provide flexibility for it, the
jabber:x:data namespace defined in Data Forms (XEP-0004)  must be used. The client can set these parameters by setting this item with this form with
This information stored in
<x/> tag with namespace
jabber:x:roster:item. Following subtags can be used for diferent types of JIDs, however client applications can make this set bigger, to implement more functions.
For all categories and types of JID allowed following subtag:
For all JIDs with category=client allowed following subtags:
For all JIDs with category=conference allowed following subtags:
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