The tag description for process-monitor is:

Process Monitor is an advanced monitoring tool for Windows that shows real-time file system, Registry and process/thread activity.

while procmon is:

Process Monitor is a free advanced monitoring tool for Windows that shows real-time file system, Registry and process/thread activity.

The two tags clearly refer to the same topic, i.e. the Windows application Process Monitor. Would it make sense to make procmon a synonym of process-monitor, because procmon was the old name and Process Monitor is the current?