Boye Circular Row Counter

I got this row-counter in my beginner crochet kit. It does not explain how to use it. How does this thing work? I can manually switch the numbers, but what is the

Boye Row Counter - Knitting Companion

Firefly Notes Stitch Marker

Row Counter Rings For Knitting - Size 12, 21.4Mm

Knitting Row Counter

The Knitter's Helper Silicone Stitch Markers

All in One 12pcs 2 Sizes Plastic Knit Counter Knitting Crochet Stitch Marker Row Counter for DIY Craft (Mixed Size-12pcs)

My Crochet , Mis Tejidos by Luna: Mis Bellos Regalos Llegaron del Intercambio Navideño con Daiana.

Boye Stitch Markers, Rings