In the Jabber
open-source instant messaging
system, indicates a relationship between two users involving the transfer of presence
information. If Alice subscribes to Bob, Alice will receive presence
messages from Bob indicating Bob's current status. Subscriptions are frequently bidirectional
(e.g. Bob subscribes to Alice as well), but need not be.
Subscriptions are managed by using <presence/> elements with a "type=" attribute of "subscribe" (to request a new subscription), "subscribed" (to acknowledge a new subscription), "unsubscribe" (to request the removal of a subscription) or "unsubscribed" (to acknowledge the subscription's removal). Subscription information is also stored as a "subscription=" attribute on roster items for a user.
In online discussion or code, the term "subscription" is often abbreviated "S10N".