This dialog is used to select a Campground Master data field (e.g. a reservation, customer, or transaction field) to be used in a parsing rule. It will open when using Browse from the Edit Parsing Rule dialog.
The Data Field list on the left-hand side will show fields that can be used for the selected Data Table (record type). It may also have "pseudo-fields", which aren't real fields in the database but can be chosen just like a real field to handle the parsed text a certain way. For instance if you choose "Number of Nights", it will actually use the given value to calculate the real Last Night field value when parsing the text.
The Linked/Relational Data Field list on the right-hand side will only have fields in it if the field you choose on the left is a relational (linked) field. For instance if you're looking at Reservation fields and select the "Customer" field on the left (which is a relational link to the customer data table), then all Customer fields will be shown on the right. When there is a list on the right, you must choose one of those fields for the selection to be valid. So for instance you could choose "Last Name" from the list of customer fields. Note that in this case it would have been the same thing as selecting "Customers" directly in the Data Table list and then selecting "Last Name" in the left-hand list -- it's just two ways of getting the same result.
Once you have selected the desired field, click OK and it will convert it into the proper Field Descriptor for the parsing rule. As a shortcut, you can just double-click on the desired field.