In short, the more I can control diameters, lengths, materials and details, the more I think I make a pen. If I do not control that, I feel like dressing a kit.
It would follow the client's requests : "Hey you who made this pen, can you make another one with a shorter but wider cap, with a centre band made of another material, with a shorter clip made of nickel silver with an S shape? And for the fountain pen please use a sightly oblique (say 5% left-foot) stub nib." The more I provide myself with the means to say yes - even if I don't make it myself - the more I make a pen. The more I answer no, the more I dress kits.
Of course, there are limits : fp feeds and nibs, refills, converters and bp mechs have to be bought I suppose.