Guild Wars 2

Hullo folks!

I have a dilemma on combo fields priority, in specific I play a hammer guardian with constant light field (due to symbol) and I would like to know if there is any explanation on how priority works for combo fields. I would imagine the last field takes priority for the blast finishers.
In practice I would like to determine whether putting fire fields down with an omni-present light field is convenient/rational.

priority is given to the first combo field laid down, until its gone. and the next field takes priority at the point, and so on.

many thanks for the clarification.