My understanding is that purchasing Concurrent users is only available to clients who already have Concurrent User licenses, not for new sales.
In addition to Registered User, you also have the option of Named User. Note that this depends on the product and in some cases on the type of user (Standard, Occasional, or Unknown) that is being licensed. Not all license models are available with all products or in all situations.
Officially they have been saying that concurrent is unavailable for quite a while. But like most things it is probably negotiable. Especially if the sales guy sees that there is a significant commission involved.
For me it depends on the product I buy and who is using it (the user definitions).
If a person is using an OpenEdge application to perform their job function they are classified as a Standard User. In this case I buy licenses on the Named User model, though Registered User (formerly Registered Device) is also an option. If such a person uses an application less than two hours a week, they are classified as an Occasional User. Though I don't deal with those so I don't remember the licensing implications offhand.
If the user does not have an OpenEdge installation, say, they're the end user of my client's web app, they are classified as an Unknown User. For those products (App Server or PASOE) we buy them on the Access Agent model.