Good question. I can only go on what I have learned so far from experienced collectors and the opinions vary enormously. A "bitsa" from what I understand is one that has literally been put together(presumably in the civilian world only, because if it was done by the military it would been stamped and be "collectable" and be of significant interest???) from parts lying around or salvaged from other defunct rifles, in order to create a rifle that effectively did not exist before.
Apparently this is quite common practice and as long as it is safe and sound then that's fine. But as far as collecting is concerned, it is seems only to be worth the " sum of it's parts" (but perhaps valuable as a de-act??).
Perhaps it would be a good idea to have a system of markings for civilian gunsmiths to add to the military ones in order to retain "service history". I imagine a rifle going through FTR might have had many parts exchanged but presumably with barrel,action, bolt, magazine, all from the same maker??????. Or would that make many FTR'd rifles "Bitsa's"?.
What exactly does make a rifle NOT interesting to a collector? I suppose the rarity of all matching original condition means collectable but perhaps for some of us the" bitsas" are MORE interesting and certainly cheaper. They are also perhaps very necessary in order for the collectable ones not to get worn out and turned into guess what.
Experienced comment for the uninitiated please!.