One-off vs. custom

I was skimming through the stylin concepts catalogue and i noticed it says "custom truck accessories" on the cover. So is that stuff actually "custom" or not? Just because its not factory, does it mean its custom? My view is that custom is basically the same as one-off becuase you have done something that you cant buy and nobody else has. So i guess my question is... can you actually buy custom? Just something to think and talk shit about...