Vertigraph's Concurrent License Manager (CLM) lets a 3-license pool float across more than 3 installed seats — a team of 5 estimators sharing 3 active licenses pays for 3 plus $149/yr per additional install.
Named-user licenses are tied to one specific user. Concurrent licenses are tied to a pool that floats; the first N users to launch the application get a license, the (N+1)th user has to wait.
Concurrent makes economic sense when estimators don't all use the tool at the same time. For full-time estimators all working in parallel during a busy bid period, named-user is usually cheaper since there's no idle pool.