⎕ID ID Number

The niladic system function ⎕ID returns a single integer which is the System ID Number.

To provide security for applications and packages written to run under APLX, ⎕ID, can be used to query the system identification number. Each APLX interpreter has a unique identity code.

This feature (used, of course, within a locked function) could help prevent unauthorised use of software by detecting the use of a function on an unauthorised system and taking appropriate action.