Microsoft Client Access Licenses(CALs)
CALs allow you to use Windows Server as a terminal server. The terminal server permits multiple connections to itself in order to provide ability to use it like one virtual machine or physical host for a larger number of users. For each connection terminal server consumes a one CAL. Parallels RAS does not replace the need for Microsoft Client Access Licenses. RDS License server is strongly required to use RAS with RDSH role.
For example, if you have 100 RAS licenses and you want to use them with terminal servers, you have to buy at least 100 CALs for it.
Without CALs you will receive the error 2048 when users are trying to open a published application or desktop.
More detailed information about CALs and how to deploy RDS License server can be found in the following Microsoft article.