The design of gimbal head machines makes them very useful for many applications, but the same features that make them useful, also make them more susceptible to a crash. The fact that the rotary axes can point cutting tools at virtually any angle, means the safe travel limits of this machine are dynamic, and change with machine position. Multiple axes are both involved in, and limited by, the definition of these limits.
The VR machines have permanent elements (see above left and right) that can potentially be involved in a crash, like the machine table, machine column, spindle head, tool changer, and machine bridge. It also has NON-permanent elements to consider, like tool length and part dimensions on the table.
The goal is to eliminate the possibility of a crash between permanent elements of the machine.