By default, tables are sorted in ascending order by this.

What’s the answer?

Record number

Explanation:

It simply can't be "data type" as there cant be more than one data type in a field. It won't be "primary key field" either as primary keys only set the field as unique. You can test this by changing the primary key to a different field. But for simplicity's sake, I did this experiment for you.

As you can see in the first and second attachments, ID is the primary key, set default by the program.

But when I set Field 8 as the primary key, shown in the third attachment, field 8 doesn't sort, shown in the fourth attachment.