Remote Desktop Services Client Access License (RDS CAL) is the actual license issued per user or per device basis to authorize users/devices to use the RDS environment. There’s a licensing grace period of 120 Days during which no license server is required. Once the grace period ends, clients must have a valid RDS CAL issued by a license server before they can log on to an RD Session Host server. You can purchase RDS CALs from Microsoft or other retailers. It is issued in 2 types one is RDS Device CALs and another one is RDS User CALs. Generally RDS User CALs are preferred if you are using MS RDS Web Client.