This is inspired both by [MENTION=23583]Yamato Nadeshiko[/MENTION]'s wonderful series on Si and on my growing frustration with the way Se types are perceived on this site. I can only really talk about the way Se functions for me, which is in the service of Fi. Please feel free to ask questions. I'll probably add more to this thread when I think of it.
Fi is the way I structure my thoughts, emotions, experiences, observations, etc. Think of a cave mouth. You enter it and underneath is a vast space filled with winding catacombs, stalagmites and stalactites, pools, and rivers. Multiple tunnels branch out and in. It's confusing to an outsider but the whole thing is meticulously organized. New perceptions are placed inside, adding to the labyrinth. Stalactites are refined by running water. Catacombs are smoothed with footprints from much exploration. It's a system of careful organization and it's very, very large. Sometimes even I get lost there.
Se is commonly perceived as a thought process and it's not. It's a way to information gather. Fi information gathers internally. Se information gathers externally. That is to say that Fi utilizes a great amount of thought about meanings and abstractions, or in other words, things that can't be touched, seen, heard, or smelled, but can be felt emotionally. There is a system of careful analysis which is helped by Ni in the ISFP. Deciphering patterns, making connections, testing them, and refining connections. Testing is where Se comes into play.
I need to reality test to figure out whether my Fi is giving me an accurate picture of the world. To figure out whether I'm organizing right. There has to be some external connection or information gathering process or I'll fall into a world made up entirely of my own mind. I've been lost there before and it really can destroy you in a lot of ways. Relying on the Fi-Ni loop means that you stop taking in new information, which is something your world view can't grow or adapt without. Se is in service to Fi because Se, or experiences, give your Fi a more accurate picture of the world, of others, of yourself, and help you to develop as a person.
Se makes an ISFP adaptable and open to new perceptions. The Fi-Se-Ni combo makes ISFPS masters at juggling and integrating perceptions, at viewing multiple things from multiple points of view. At understanding others, understanding why they think the way they think, and taking that into consideration, both to accomodate others and to expand their own views. ISFPs are keenly aware of who they are and who they could be, and the possibilities inherent in that.
The world is constantly changing. Nothing is set in stone. No one point of view is right, and ISFPs have an inherent awareness of the way multiple seemingly contradictory concepts exist at once.