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Most file systems have methods to assign permissions or access rights to specific users and groups of users. These permissions control the ability of the users to view, change, navigate, and execute the contents of the file system. In some cases, menu options or functions may be made visible or hidden depending on a user's permission level; this kind of user interface is referred to as permission-driven.
Two types of permissions are very widely used: traditional Unix permissions date back many decades to the earliest days of Unix. They are universally available on all Unix and Linux derived platforms. Access Control Lists (ACLs) are more recent in origin and are universally used on Microsoft Windows based file systems where the file system supports user permissions (mainly NTFS and ReFS), and are also now commonly used and widely available in most common Unix and Linux based systems, although not necessarily all. They are generally capable of far more detailed fine-tuning of permissions than the traditional Unix permissions, and permit a system of access control which traditional ACLs cannot provide. On Unix and Linux based systems, the standard type of ACL is that defined by the POSIX standard (POSIX ACLs) but other variants exist such as NFS v3 and v4 ACLs, which work slightly differently (NFSv3 ACLs or NFSv4 ACLs).
Where multiple systems are available within the same operating system, there is usually a way to specify which will be used for any given file system, and how the system should handle attempts to access or modify permissions that are controlled by one of these, using commands designed for another. The usual solution is to ensure at least some degree of awareness and inter-operability between the different commands and methods.

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