【Super User】: The root user is the default super user account in the Linux system and has supreme and complete permissions on the host, similar to the Administrator user in the Windows system. Only when performing system management and maintenance tasks, it is recommended to use the root user to log in to the system. It is recommended to use an ordinary user account for daily transactions.
【Ordinary User】: Ordinary user accounts need to be created by the root user or other administrator users, and their permissions are subject to certain restrictions. Generally, they only have full permissions in the user's own host directory.
【Program User】: When installing the Linux system and some applications, some specific low-privilege user accounts will be added. These users are generally not allowed to log in to the system, but are only used to maintain the normal operation of the system or a certain program. For example: bin, daemon, ftp, mail, etc.

1:MANHave a look:

R，-系统 创建一个系统帐户。 系统用户将在/etc/shadow创建他们不会变化的信息，它们的数字标识符在sys_uid_min-sys_uid_max范围，在/etc / login.defs定义的，而不是 对uid_min-uid_max（和他们的创作是同行 组）。 注意，不会为用户创建一个主目录，不管在/etc/login.defs的默认设置 （create_home）。如果你想要一个的话，你必须指定的M选项将创建一个系统帐户的主目录。

2: The following is the difference from not adding -r.

1: First of all, the uid and gid assigned to it are within the range of sys_uid_min-sys_uid_max (my computer is 201~999), not greater than 1000.
2: The difference is the above difference, which actually reflects the structure of our system. I don't know the function of distinguishing user identities based on uid.