Currently a derived field happens at the end of processing. You derive a field from different input.
Priority fields also happen late in the game.
I believe there is a situation when finding a value for a Measurement's value_as_number where you might have a priority set for extracting what would be value_as_string from either an attribute or the #text. To get to value_as_number, you'd do that, and THEN have a derived field that uses a function to cast the winning value_as_string field/value over to value_as_number.
Currently a derived field happens at the end of processing. You derive a field from different input. Priority fields also happen late in the game.
I believe there is a situation when finding a value for a Measurement's value_as_number where you might have a priority set for extracting what would be value_as_string from either an attribute or the #text. To get to value_as_number, you'd do that, and THEN have a derived field that uses a function to cast the winning value_as_string field/value over to value_as_number.
Tread carefully around that code.