Hey there folks

So I have a couple questions regarding how Soul Trap, Soul Gems, and the Soul Cairn all fit together. So here's what I know:

If something is hit with Soul Trap, and killed before it wears off, their soul will be trapped in a Soul Gem and then eternally confined in the Soul Cairn. The thing that confuses me is what's in the Soul Gem? I mean, I know it's purported to be a soul. It's labeled as such in game menus and in texts, but if that's the case, is the target held in limbo until the Soul Gem is used in an enchantment, and then sent to the Soul Cairn?

The reason I asked is because I remembered last night that the Soul Cairn can fill up your Soul Gems at various crevices and the like littered throughout the area. What is being harvested to allow you to get these souls, and why don't they have any appearance, as opposed to the other souls you find scattered about? Do all souls head straight to the Soul Cairn upon death (as is implied by talking to some people in it), only to lose their forms once the Gem has been used up, or is there something different and unique in the Soul Gems that is taken away from the victim at the time they are killed?

The only thing I can really come up with that could be taken and put inside the Soul Gem to fuel an Enchantment is the potential life energy of the victim, similar to the Weeping Angels in Doctor Who. In effect, every successful Soul Trap is a bargain with the Ideal Masters: They agree to give you a means of taking a power you otherwise wouldn't have access to, and in exchange, they are given the soul of a person they otherwise wouldn't be able to get.

This is the best I could come up with, but I was wondering if there was an official answer and explanation that I otherwise missed.

I dispute that the soul goes to the Soul Cairn. Some people (not including me) have taken that impression away from Dawnguard, but it doesn't make any sense in the larger scheme of things. What, the Ideal Masters have more control over soul gems than the inventor of soul gems, Molag Bal, or the god of necromancy and major component in the creation of many a black soul gem, Mannimarco/the God of Worms/Revenant, or any of a number of other soul grabbers, from other Daedric Princes to general petty necromancers to soul-snaring Sload?

What you capture in the soul gem is a soul. If you'd like, you can give the soul to the Ideal Masters as a gift or trade good, but it doesn't go to them by default.

I take the souls pouring from soul crevices as compressed storage of sorts. The Ideal Masters have been operating for a long time; eventually, they'd run out of space. ESO's Coldharbour includes strange, milky white water, which I personally take to be another solution to the issue of soul storage.

Would the soul not be the life energy? Seems like some twisty logic.

Again, I see no reason to think that captured souls automatically go to the Ideal Masters. If that happened, we would have heard about it a long, long time ago. We would have heard about N'gasta negotiating with them back in Redguard, or mortal Mannimarco dealing with them in Oblivion, or any of a number of other things. It does not make sense that the Ideal Masters collect every captured soul by default: The souls must be delivered to them.

Theory-time:

I've always connected the whole soul business with what we know of the dreamsleeve system and the way Daedric realms work. A Daedric Prince (and, it is implied, lesser Daedra as well) gathers up raw creatia and uses this to build his realm, which is also himself. What appears to happen in the Soulsleeve (which some have theorized is in the Aetherius, origin of creatia) is that the deceased soul is recycled and purged of his acquired memories. The soul is then deposited again on Nirn where it can start producing new memories. From what does it make these? Creatia, recycled and beamed down again from beyond the beyond, used in the construction of a little thought-world much as a Daedra would make a physical body or realm from the stuff.

So, what you're doing with a soulgem is trapping a soul, and using it as a little engine to attract and focus creatia in an object, in the process imbuing it with new attributes (making it more "real" than before). Presumably, the process releases the soul to return to the cycle. Except, of course, if you sell this soul on to one of various interested parties: both the Daedric Princes and the Ideal Masters have their realms to maintain, and a vault full of creatia-attracting souls would help quite nicely with that. Of course, they could rig the game from the start, teaching mortals a very specific way of soul-trapping that ensures the soul will go to their realm when used (Molag Bal, Mannimarco's black soulgems).