How a USL Compares to a CAL

A User Subscription License (USL) for a Microsoft Online service is similar to a Client Access License (CAL) in that it licenses client access to Microsoft server software. However, it differs in a few important respects.

USLs are available only per-user while CALs can be purchased per-user or per-device. This puts Exchange Online at somewhat of a price disadvantage compared to on-premises Exchange in situations involving call centers and other scenarios where two or more employees share the same client device during the course of a day.

USLs license access to both online services and the equivalent on-premises server software. Besides allowing a user to access Exchange Online, an Exchange Online USL includes the same set of rights to access on-premises systems as do the Exchange Server Standard CAL and Enterprise CAL. In other words, the USL is equal to a Standard CAL plus an Enterprise CAL for on-premises use. While in the case of Exchange, users are unlikely to straddle both hosted and on-premises systems (i.e., have a mailbox in the cloud and a mailbox on the on-premises system), the dual nature of USLs can be handy in hybrid deployments (where some mailboxes are in the cloud and some on-premises) or during a migration period.