Hello,

contact sort fields are used to sort contacts in a specific way:
first, sort field 1 of all contacts is tested when ordering contacts. When contacts have the same sort field 1, then the sort field 2 is tested and so on.

Therefore, for best results, the contact sort field 1 should be the last name of the contact, the contact sort field 2 should be the first name...

That is why you cannot choose which sort field to use because they are all used if non empty.

Sort fields can contain any kind of information, but you can only sort by name OR sort field, not both.

If you want to sort by name first then by a second sort field, the name must be added to the first sort field, then whatever other info you need in sort field 2.