Start with your machine’s accessory list, then choose a foot for the stitch you want to sew. “Snap-on” describes an attachment method; it does not promise that every snap-on foot fits every machine.
Keep the foot, its holder or ankle, and any required attachment together when comparing listings. For a branded feed system, use the accessory instructions for that system.
Match the actual sewing task
An open-toe foot improves the view around appliqué and decorative stitches. A walking or dual-feed attachment helps handle layers. These solve different sewing problems and may require different settings.
For example, PFAFF tells users to engage IDT after fitting its Clear Open Toe Foot for IDT. Husqvarna Viking’s Interchangeable Dual Feed Foot uses changeable soles and has a separate list of supported machine groups.
Check needle clearance before powered sewing
Switch off before changing the foot. Fit it as the manual shows and select the intended stitch. Check the needle path with the handwheel before starting under power. A narrow opening must not be used with a stitch that swings outside it.
Treat stitch width as a limit
Singer lists a maximum 6 mm stitch width for the 4423. That describes the machine; it is not a promise that any 6 mm foot or any narrower foot is suitable. Check the selected foot’s instructions, attachment and allowed stitch together.