This is a type of commercial software license. It is not a software product itself. A CAL is needed for each user or device that connects to the server to use its services, which include file sharing, printing, or authenticating against a domain controller. It is a license that allows a user or device to legally access server software services. Microsoft uses this model for their server products, such as Windows Server, Exchange Server, and SQL Server.

An RDS CAL (Remote Desktop Services Client Access License) is a license that allows users or devices to connect to and use a Windows Server with the Remote Desktop Services (RDS) role installed. This license gives the user or device the legal right to access the server's published resources, including virtual desktops and applications, through the Remote Desktop Protocol (RDP).

There are two types of RDS CALs: Per-User RDS CAL and Per-Device RDS CAL. Choosing the right one depends on how your organization's users and devices connect with the server.

A Per-User RDS CAL is a type of Remote Desktop Services Client Access License assigned to a specific user rather than a device. This licensing model works well for organizations where one user needs to access the remote desktop server from multiple devices.

Here’s who can use a Per-User RDS CAL and when it is most useful:

• Employees who work from multiple locations or devices can benefit the most. For instance, a user might access the remote desktop from their work desktop, a personal laptop at home, and a smartphone or tablet while traveling. A single Per-User CAL covers all of these connections.
• Organizations with flexible or remote work policies gain from this model. Businesses that let employees work from various locations and devices can save on licensing costs by using Per-User CALs.
• Companies with more devices than users find this model cost-effective. If you have more devices than users, you only need to buy a license for each unique user, no matter how many different devices they use to connect.
• Users in an Active Directory (AD) domain environment can use Per-User CALs since they are assigned to a user in Active Directory. While Per-Device CALs can work in a workgroup environment, Per-User CALs generally need an AD domain to work well and to be monitored by the licensing server.

A Per-Device RDS CAL (Remote Desktop Services Client Access License) is a license assigned to a specific device. It allows any user who uses that device to connect to a Remote Desktop Services (RDS) server. This licensing model works well in certain work environments and scenarios.

Here’s who can use a Per-Device RDS CAL and when it’s the best choice:

• In a factory, a call center, or a hospital, one computer may be used by several employees throughout the day, such as across three different shifts. Instead of buying a separate Per-User CAL for each person, you only need one Per-Device CAL for that computer.
• Libraries, hotel kiosks, or school computer labs. Many different people use these devices, and a Per-Device CAL makes licensing simpler by linking it to the machine instead of the ever-changing group of users.
• If you have 100 employees but only 50 computers, a Per-Device model is more cost-effective. You only need to purchase 50 CALs instead of 100.
• Per-Device CALs can be tracked and managed by the license server without needing the devices to be part of an Active Directory domain. This makes them a good choice for simpler network setups without a domain.
• Unlike Per-User CALs, which are not always enforced, the license server for Per-Device CALs can maintain the limit. If you have 5 Per-Device CALs and a sixth device tries to connect, the server will refuse the connection. This can help with compliance and resource management.

Per-Device RDS CALs can be tracked from Licensing Manager. Per-User RDS CALs can be tracked only in the Active Directory environment, they cannot be tracked in a workgroup environment.

Per-Device lets you revoke up to 20% of RDS CALs. This is not possible with Per-User.

Temporary RDS CALs are part of the Per-Device RDS CAL licensing model. They are not a separate product to buy. Instead, they are temporary licenses issued by a Remote Desktop Licensing server.

Here's how they work:

1. First Connection: When a new device connects to an RD Session Host server in Per-Device licensing mode for the first time, the server checks with the RD Licensing server.
2. Issuing a Temporary CAL: The RD Licensing server provides a temporary RDS CAL to that device. This lets the device connect and allows the user to start their session, even if a permanent CAL hasn't been assigned yet.
3. Validation and Conversion: After the user signs into the session, the RD Session Host server tells the Licensing server to mark the temporary CAL as validated. The next time that device connects, if a permanent Per-Device RDS CAL is available on the license server, the temporary CAL is upgraded to a permanent one.
4. Expiration: Temporary RDS CALs usually expire after 90 days. If the device doesn't get a permanent CAL from the license server within this time (for example, because no licenses are available), the temporary CAL will expire. The user will then be unable to connect to the session.

Key Points about Temporary RDS CALs:

• Temporary CALs only work with the Per-Device licensing model. The Per-User licensing model does not include temporary CALs.
• The RD Licensing server automatically issues temporary CALs. You do not need to create or assign them manually.
• The temporary CAL provides a grace period. This gives you time to install and activate permanent licenses after the initial deployment, without interrupting user access.
• When a temporary CAL expires, users on that device will be disconnected from their session after a warning period, which is typically 60 minutes, until a valid permanent CAL is available.

Administrators should ensure they have enough permanent RDS CALs installed on their licensing server. This helps cover all devices that will connect to the RDS environment and prevents service disruptions.