What does the 4-in-1, 7-in-1, 9-in-1, etc. actually mean?

Most Instant Pots are categorized by referring to the number of appliances a single Instant Pot can replace.

For example, the Duo 7-in-1 replaces the following appliances: pressure cooker, slow cooker, sauté pan, steamer, rice cooker, yogurt maker and food warmer.