Explanation:

Site User Visibility in Sharing Settings controls whether external (community) users can see and search for each other. When it’s turned off, community users cannot find or view other external users in lookups, chatter, or user lists. Internal user visibility and profile updates (A and C) remain unaffected because Site User Visibility applies only to the portal’s external user base.

This setting is crucial for privacy in a community—especially a Partner Community—where partners should not necessarily see every other partner’s profile. Turning it off locks down that level of discovery.