It is interesting how these terms remain all but undefined. It seems that it all changes from person to person and seller to seller. Igor Shirogorov says his knives are not custom even though he and his brother are the only hands involved in making them... but Neil at TNK says he would definitely call them custom but doesn't because the maker doesn't. Then there are aspects that could change the title but not really change the knife. Like a Lisa's Pick Sebenza. It has a one of a kind, custom design on a "production" knife. Does that make it a "Custom" knife? I don't believe CRK thinks so. Then we have makers like Darrel Ralph... I see little to no difference between his customs and productions yet somehow there is a line drawn between the two.
With instruments there is a "Luthier" who makes, say guitars. With knives there is a Knifemaker or Bladesmith who makes knives. A fancy, one man made, guitar is considered a "Luthier made" instrument. It does not matter if it is totally unique or the same as every other of that model. Maybe we should have classifications like:
Bladesmith made - one man made it.
One of a kind - nothing else like it (don't hold your breath)
Midtech - Small batches, small(er) shop <10 employees
In-House Production - Everything but (at least) the screws are made in the same building as the owner/proprietor's office.
Outsourced Production - in this case I want to know where and why.
Custom - ...is kind of a bad term. I mean I could change the edge angle from the factory and in a way it would be a "custom" knife.