Every furnace is unique and has its own identification number. This gives you and a technician all the basic information they need to find parts, wiring diagrams and possibly owner’s manuals for your unit.
Typically, you model number is found on a sticker and depending on your unit could be found in a few different places. The first spot to check is on or near the front inside door panel of your furnace. The second place to check is on the inside wall panel of your furnace, usually found closer to the bottom. For a/c units they are usually on the outside of the unit. The model number of a furnace IS NOT found on a blower assembly, motor or any other part, as those are numbers specific to that part only.
You model number can start with either a number or a letter and can include a model number (M/N) and a series number. Your serial number (S/N) is typically found on the same sticker but most technicians will not need this, it mostly for the manufacturer. The unfortunate part of the model number placed on a paper sticker is, whether inside a furnace or outside on an a/c unit, over time the heat and light fade the tag over time and it becomes unreadable.