Some documentation (programming) refers to these terms when there is some kind of limitation on CPU usage, for example:
Background tasks are limited by the amount of time use of the wall-clock .
Background tasks are limited to 30 seconds of using the wall-clock .
Without understanding what exactly these terms mean, it is difficult to tailor software so that it does not extrapolate the recommended CPU usage.
CPU Time is the time the processor spends to perform a task.
Wall Clock Time is all the time passed to execute a task, so if there are interruptions in the activity execution the wall clock does not stop counting, but the CPU time is not considered.
Interrupt can occur because the processor splits its usage with other threads or because it needs to wait for a device external to the CPU to respond or the algorithm is on hold without executing anything.
This is a way to differentiate whether the threshold you can use uses one criterion or another. If the limit were for the CPU time it could actually run for minutes, hours or even days, since most of the time it is not running anything.
