A good rule of thumb I have found is if you need to make up rules to explain how something works it likely doesn't work that way.
Example we can assume Instant enemy grants FE bonus works with any spells that use Fe And gives an AC the Fe benefit.
Anything else require a lot of rules extrapolation not in the spell.
Example I have Fe of 6 vs outsider (water)
I cast IE do I now assume my target cannot be crit I cant flank him... or any number of quesquestions.
It seems obvious to me none of these assumptions are correct as they all require slot of information not in the spell.
A good rule of thumb I have found is if you need to make up rules to explain how something works it likely doesn't work that way.
Example we can assume Instant enemy grants FE bonus works with any spells that use Fe And gives an AC the Fe benefit.
Anything else require a lot of rules extrapolation not in the spell.
Example I have Fe of 6 vs outsider (water)
I cast IE do I now assume my target cannot be crit I cant flank him... or any number of quesquestions.It seems obvious to me none of these assumptions are correct as they all require slot of information not in the spell.
It doesn't require any extra extrapolation. You must simply ask yourself the question, does my character treating this enemy as a type allowed by IE change how I interact with it (note how it does not effect how it interacts with you). If the answer is yes and you make that change, then that change takes place.
Hence why the spell says for all purposes, and not just "for purposes relating to your favored enemy bonus".