Quote from PAPA ROACH:
We trade both ways and suprisingly do alot of trades over AOL IM. IM lets you work on multiple deals simultaneously. ICE is just a trading interface on the physical side, the deals are bilateral. You just do not know who the counterparty is until you execute. It has a credit setting function that allows you to turn counterparties on and off, and also lets you set daily and/or monthly limits to each counterparty. The daily spot market trades actively from about 7 a.m. to 10:30 a.m. CST. Trades continue but liquidity dries up and it stops at 11:30 a.m. at which time next day timely nominations (scheduling) for the pipelines is due.
There is alot more to it than just the little I layed out, but that is generally how the next day spot market on physical natty works.