Hi @Yeung, Patrick CP ,
CAL is a license that gives a user the right to access the services of the server. It is divided into two types: User CALs and Device CALs.
"CALs are required for each person or device accessing a SharePoint Server" means, you can buy User CALs or Device CALs, you need to have one of them.
The following are the applicable scenarios of the two CALs, you can choose the CAL that is more suitable for your situation.
**1.**User CALs****: With the User CAL, you purchase a CAL for every user who accesses the server to use services such as file storage or printing, regardless of the number of devices they use for that access. Purchasing a User CAL might make more sense if your company's employees need to have roaming access to the corporate network by using multiple devices, or from unknown devices, or if you simply have more devices than users in your organization.
**2.**Device CALs****: With a Device CAL, you purchase a CAL for every device that accesses your server, regardless of the number of users who use that device to access the server. Device CALs may make more economic and administrative sense if your company has workers who share devices, for example, on different work shifts.
Reference: Client Access Licenses (CAL) & Management Licenses | Microsoft Volume Licensing
In theory, you buy CAL licenses for each user who can access the SP2019 site at the same time.
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