With few exceptions, when you are presented with a table containing a list of fields, you can choose to process the selected fields or to process all but the selected fields (called an inverted set). You change the mode by clicking on the button at the right of the table's header bar. The table's field column displays the current mode.
While it is largely your choice as to which method you use, there are some differences.
When processing selected fields, there must be at least one field selected. When processing all but the selected fields, there need not be any fields selected. When no fields are selected it means for all fields. This mode will handle the case of additional Custom Fields being added at some point.
When actions are exported, all referenced custom fields are exported as well. This may not be desired. For example, let's say that you want to trim all fields. If you select all fields and set the mode to process selected fields, any selected custom fields will be exported. However, if you clear all field selections and set the mode to process all but the selected fields, no custom variables will be directly referenced. Unless custom variables are directly referenced elsewhere, they will not be exported.