What is X-Mobile, instead of just mobile in contacts
There are two entries for the given number/person, one below the other on the same page. It is the same number, but the one on the top is called mobile (in small letters), and the one below it is named X-Mobile.
Also, there are two entries, for example home and home, or mobile and mobile or work and work(both in small letters) and there are a few contacts with numbers named X-Mobile.
What is this X-Mobile, and how to keep everything just mobile, home or work? (By the way, these days rarely anyone has a stationary home number.)
iPhone 11