Not a DT teacher, but did A-level DT and have just finished a Design degree…
it is rather simple really,
Permanent fixing methods (eg. welding) cannot be undone without damaging the parent material.
Semi-permanent methods (eg riveting) can be undone usually by destroying the fixing method (in this case by drilling out the rivet), but the parent material is left intact.
Non-permanent (eg. nuts and bolts) can be undone and redone a number of times without damaging either the parent material or the fixing.
This is not a ‘text-book’ answer, rather one off the top of my head, but as far as I’m aware it should suffice.