Whoever is in charge of the Volunteer Opportunity is designated as the Opportunity Coordinator.  

As a matter of good business practice, the opportunity coordinator field lookup only finds contacts that have had portal access granted.  

Typically Opportunity Coordinators should be either Partner Staff or Volunteer Leaders (who have access to manage volunteers and report attendance through the Customer Portal).

For partner projects, this is typically someone on their staff, and hence they are not a volunteer.  So they are strictly designated as the Opportunity Coordinator.

The Opportunity Coordinator who is designated in the Volunteer Opportunity record is considered the 'default' coordinator for all occurrences created in relation to it.

The default Opportunity Coordinator can be edited at the Occurrence level if different people lead different occurrences.

When the Opportunity Coordinator you assign to an occurrence has a volunteer leader profile, and it is an affiliate managed opportunity -- the system automatically creates a connection (sign-up) for this volunteer, and designates this person as the Volunteer Leader (VL)

You can assign additional Volunteer Leaders to an affiliate-managed occurrence - but only one is designated as the Opportunity Coordinator.