With the Select Fields window you can select a number of fields from a background. So far this window is only used for specifying the structure of a line in a card list field, i.e. the columns of the list, which fields to include and which not (see Edit Fields and Controls window).
You use multiple selection in this window: To add a field, click while holding down the "Control" key.
The order of the fields is the order of them within the background in question. It does not matter which field you select first, and which second - the order of the fields in the list will determine the order of the fields in the resulting field list. (For a different field order, write the field list yourself "by hand" on the corresponding field of the Edit Fields and Controls window.)