The chicken parm grinder is what New England calls a plated dinner that has been talked into a roll. The name is the regional tell: in Connecticut, Rhode Island, and Massachusetts the long sandwich is a grinder, and the grinder tradition there leans hard on one move, which is running the built sandwich back under heat until the cheese blisters and the roll toasts. That finishing pass is the defining thing. A chicken parm grinder is not a cold sub with hot filling dropped in; it is assembled, capped with mozzarella, and fired until the top browns, which is closer to how the plated chicken parmigiana behind it is made than to how a deli builds a sandwich.
The craft is a moisture-management problem the grinder solves with heat and timing. A breaded chicken cutlet is fried for a craggy shell, sauced with marinara, blanketed with low-moisture mozzarella, and set on a split roll. Every one of those components is wet or about to be, and the enemy is a roll that surrenders before the last bite. The New England answer is to toast the roll first so it has a sealed, crisp interior face, keep the marinara thick rather than loose so it coats instead of floods, and put the cheese against the cutlet so it glues the stack as it melts under the salamander. The crust on the cutlet is the only crunch in the sandwich, so the bread is chosen sturdy and the sauce is kept tight specifically to defend it. Done fast and hot, the grinder holds; done slow with thin sauce, it collapses into the roll.
The variations stay inside the toasted-roll frame. The veal parm grinder runs the same build with a pounded veal cutlet; the eggplant parm grinder takes it meatless with fried eggplant standing in for the protein; the meatball grinder shares the roll and the salamander pass with a different filling entirely. The chicken parm hero is the same sandwich under New York's name and New York's crusty Italian roll, which is a regional distinction worth its own treatment. Each of those is a codified build with its own rules, and each deserves its own article rather than being crowded in here.