http://www.jempeg.org/tagNames.html -- at the bottom is the search syntax (same as Emplode basically)

Soups will update dynamically as you add music.

Currently you can't edit the criteria ... Well .. I take it back, you can't edit the criteria through the UI. There are two different types of soups. The jEmplode-only soups appear in .jempegrc and can be edited there (there's a message in the original Prerelease announcement thread that talks about the format). For on-empeg soups, the top level soup node has a new tag in it called "soup" whose value is the same as the soup values in .jempegrc. You can edit that value with the Advanced Tag Editor tab of the Properties dialog for the playlist. The soup won't actually update until the next time you sync though.