This is a vanity feature. You can flaunt your friends with this.
Your identification keeps some data for each person you communicate regularly with in its peer data structures. It can also store settings for places or groups.
Expose is one of those. It specifies whether you'd like a person, place or group to appear listed in your profile. This is equivalent to a top friends type of listing in typical social network applications.
You can specify the degree of exposure of a UNI using the /expose command. 0 is to never show and 9 is to show always. In between values make it more or less likely depending on the degree of trust you have for the person who is requesting the profile from you.
The /show expose command tells you who your top friends are. /set friendsexpose and /set groupsexpose set the default degrees of exposure.
See also trust, display and notify.
The /cancel command lets you delete the entire record for a peer.
P.S. psyced currently doesn't implement exposure maths correctly.