Linux UIDs (user identifiers) are numbers that are used to identify users and groups on a Linux system. Each user and group has a unique UID and GID (group identifier) respectively. The UID 0 is always reserved for the root or superuser account, which has full privileges to access and modify the system. The UIDs lower than 100 (or 1000 on some modern systems) are typically reserved for system accounts, which are used by various services and daemons that run on the system. These accounts are not meant for human users, but for specific purposes such as managing files, processes, network, security, etc. For example, some common system accounts are bin, daemon, mail, sshd, etc. The UIDs higher than 100 (or 1000) are usually allocated for regular user accounts, which have limited privileges and can be created and deleted by the system administrator. The system accounts are defined in the /etc/passwd file, which contains the username, UID, GID, home directory, shell, and other information for each account12345. References: 1: Linux User Management - Tecmint 2: What are the well-known UIDs? - Stack Overflow 3: user ID less than 1000 on CentOS 7 - Unix & Linux Stack Exchange 4: Recommended GID for users group in Linux (100 or 1000)? - Unix & Linux Stack Exchange 5: What is the conventional purpose of Linux UIDs that are lower than 100? - VCE Guide