Depends on who you are talking about. Ideal can be actually "ideal", and premium can be a solid class 1, (as in Birks/Tif. etc...or ideal can be "this is our best shitty cut" and premium can be "this is our second best shitty cut".
Of note: Peoples/Zales - have "good, better, best" bfrands which mean: nasty, nastier, nastiest. Their signature cuts are: "not quite as nasty - or "fa-nasty" which means "we've added a whole bunch of extra facets so this nasty stone is now both fancy and nasty..." Or "fantasy"....
Bottom line: you have to know the merchant to know the significance of their brands
And in case you have not noticed - most diamonds sold in the US and Canada are crap-o-la.