Under Windows, this parameter specifies the user number that is used by the component file system to control file sharing and security. If you wish to share component files and/or external variables in a network it is essential that each user has a unique aplnid parameter. It may be any integer in the range 0 to 65535. Note that an aplnid value of 0 causes the user to bypass APL's access control matrix mechanism.

Under UNIX, the user number is obtained from the Operating System (UID) and aplnid is not used. If the user is "root", APL's access control mechanism is ignored.

When a user creates a component file, his user number is recorded in the file to identify him as its owner.