A toilet installation is constrained heavily by one single dimension: the rough-in distance measured from the finished wall to the center of the waste outlet in the floor. That one single measurement narrows and constrains the choice of toilet pan more than any other factor. Standard rough-ins in North America are 12 inches from the wall, but older houses might have 10-inch rough-ins, and modern renovations sometimes land at 14 inches. A toilet pan designed for a 12-inch rough-in will literally not fit into a 10-inch opening. Installing the wrong pan for the wrong rough-in distance does not work well and creates problems.
Toilet units come in three primary formats: one-piece units, two-piece units, and wall-hung units. One-piece units are heavier and less portable, but they are a single rigid monolithic object with fewer joints and seals to potentially fail. Two-piece units — with cistern and pan separated — are lighter and easier to transport and install, but they have a critical joint between the cistern and pan that has to seal properly. Wall-hung units need a concealed cistern mounted high and a structural support frame strong enough to carry the full weight and the repeated impact loads of use. A wall-hung unit saves valuable floor space and makes floor cleaning much simpler, but it demands robust framing that can support it and requires a different maintenance approach than a traditional floor-standing pan.
Seating the toilet pan correctly is about the wax ring seal and the bolts that hold and anchor it to the floor. The pan has to sit solid and stable on the floor, not rocking on high spots or being unstable. If the floor is not flat underneath, the pan might not seal properly and water will leak down into the subfloor structure and joists. Once the pan is set and sealed, it is sitting on whatever floor finish was installed first — tile, vinyl plank, or concrete. Removing an old toilet reveals what is underneath that finish, which might be stained, damaged, or deteriorated from years of moisture exposure.