Collapsed Mast Height refers to the height of the mast when a stacker’s forks are at or near its lowest point.  Sometimes called “Overall Height

The reason why this information can be crucial depends on your application of course.  For instance, if you need a stacker that fits through a doorway then the width and overall (or collapsed mast) height would be very important to know.  Since doorways on average provide about a 79” clearance it would be wise to make sure the collapsed mast height of a stacker falls conservatively short of that height.