This is highly subjective, tough to differentiate and really dependent on the individual. For me, I'd say there are three classes.

Customized/personalized/tuned, etc.. These are guns that have been tuned, restocked, maybe replacement parts like basepins, springs, sights, etc.. No serious modifications. Lots of stuff that the average person can do at home. Like this Colt SAA .38-40, tuned and stocked by Tom Sargis.



Mild custom. These are guns that have some modifications only a gunsmith can perform. Perhaps some advanced tuning like reaming throats, recutting forcing cones and crowns, coupled with other work. New sights that are not direct replacements. Metalwork like checkering or grip frame modifications. Maybe even simple caliber conversions would fall under this grouping but usually something that retains a majority of factory parts. A gun that is obviously not as it was from the factory but still recognizable. Probably upper end of the genre, this Bisley is what I consider a mild custom. Factory barrel, cylinder and chambering but with an action job, recut forcing cone and crown, minimum headspsace, freewheel, custom front sight, #5 basepin, Turnbull finishes and claro walnut by CLC.



Full custom. One that has been subjected to serious modifications. New barrels, particularly the octagon, ribbed, banded or ovate variety. New cylinders, particularly oversized five and six-shot. Although some five-shot conversions utilize the factory barrel and these would certainly still be considered "full custom". Standard and oddball chamberings. More involved metal work like topstrap conversions, grip modifications, anything involving welding and parts fabricated from scratch. Particularly if one-off. This is probably the closest I have to a full custom. An old flat-top rechambered to .44Spl, PacNor barrel, custom sights, full tuning, custom basepin, custom finishes and restocked in micarta by CLC. The .500 Jack is building for me would be the upper end of "full custom" with an octagon barrel, Bisley conversion, one-off custom basepin and front sight, some welding, ivory, engraving and Turnbull finishes.