I have been asked a few times now as to my opinion on whether Ross Sapotes are a variety of Pouteria campechiana, the canistel and whether seedlings are a new types that shouldn't be called Ross Sapote any more. Ross Sapote should be pumpkin shaped?
In the absence of a genetic analysis and knowing the definition of species is rubbery and changing, especially for fruits in cultivation we can still make an informed observation. The seeds are distinctive and the foliage is recognisably different. The evidence is that Ross and Canistel do not cross and that perhaps is the key. Closely related species can often hybridize and if these two cannot it suggests distinctiveness so yeah it looks like they are different species.
Is a Ross Sapote seedling still a Ross Sapote? It depends on how true they are and how significant a character small changes in fruit shape are. Jacks, durians, many cultivated anoona and citrus are not super dooper true and calling them seedlings of or naming a new variety is probably justified at times. That is within reason as thousands of new varieties with tiny variations wouldnt work. I think Ross is true enough from seed to still be called Ross and the difference being a slight variation in fruit shape (which is not a great diagnostic character) could mean it is best to call them Ross Seedling so to reduce confusion.