As I remember normally after one dies (on/off plane, wherever), he becomes a petitioner (of course) on the plane of his deity or alignment, but his memory and soul go somewhere else.
I can't remember where the 'somewhere' exactly is, only got the vague impression that one's memory goes to the astral and the soul (?) goes to various places on the plane where souls are classified according to their reasons of death (like in Fire & Dust).
Anybody cares to help a clueless out? 
Question: What is a memory core?
What is a memory core? That sounds interesting.
It's all the memories of a dead person, existing as an incorporeal cloud of thought in the Astral Plane (though I've seen them described more evocatively as looking like the dead person's corpse, drifting in "memory sinks" in the Astral void, able to speak to those who find them). Souls shed them on their way through the astral conduits (sometimes petitioners remember some things, but for the most part their memories exist in the memory cores).
When you cast a speak with dead spell, it's the memory core you're contacting, not the petitioner.
Thanks...
I really don't know where the 'soul classification' idea came from in the first place...maybe it only came from Fire & Dust and got mixed with what I read from the setting.
Now it's lot more clear.
Heh, the idea of "memory sinks" and the idea that memory cores might look like the corpses of the dead comes from Fire and Dust, actually.
But astral memory cores of petitioners, and the idea that they're what speak with dead contacts, is from A Guide to the Astral Plane.
Rip, don't the memories of those who pass through the River Styx (i.e., evil petitioners) literally end up as memory fragments at the bottom of the Styx?
I could have sworn I read about this in one canonical source or another. Hydroloths, being immune to the Styx, collect the memory fragments from time to time.
Edit: Found it! Hellbound. It's called the "sediment of thought" in that source. A question: Do ALL evil petitioners pass through the Styx, or is the "sediment of thought" in the Styx created by the living only? Do the memories of evil petitioners end up in the Astral even before they get to the Styx?
Yeah, memory cores end up in the Astral while in the soul passes through the Astral Plane, before they ever get to the Outer Planes.
Petitioners can acquire memories after they form in the Outer Planes, though, and can certainly lose them in the Styx, as can mortals, fiends, celestials, and anyone else. That's where thought-sediment comes from.
The petitioner is the soul. If they have a deity or faith they look to above all others, their pantheon determines where souls/petitioners go - usually either the realm of their deity or a collective underworld. Those with no paricular deity or pantheon or who revere a philosophy rather than a deity go to the plane that best matches their alignment.
Memory cores end up in the Astral plane.