So I've been looking at the different Daemon Weapon attributes and from looking at the Stream of Corruption attribute it has a number of weapon qualities including Stream but I cannot find a description of what this does.

Good question. There is no such thing as the Stream Special Weapons Quality, nor has any WH40kRP book that I know of ever had one.

The only thing I can think of is that there's a flamer in Rogue Trader that is described as sending a straight stream instead of a cone as part of it's Spray (then still called Flame) Special Weapons Quality.

I'd go with that, saying that the Stream Special Quality shoots in a straight line, instead of a cone.

That's a highly questionable interpretation, though, it's very personal on my part. RAW, it means nothing, afaik.

My groups interpretation was someone wrote stream, meant spray. Typed in spray and forgot to delete stream.

Looks like a mistake to me. Appears no where else in the book as far as I can tell. It might have been an early name for the Spray quality, and it was left in by accident.

BYE

whatever you do, don't cross the streams.