Multi-tier application and IBX

Hi,

Now I develop multi-tier application where the application server use
IBX to connect to IB Database. I not understand this note in Delphi
help:

"Note: In applications that connect an InterBaseExpress dataset to a
client dataset, every query must be in its own transaction.  You must
use one transaction component for each query component."

What is the reasen of this note? Why I must have separate
IBTransaction for every IBQuery?

Tnx,

Miro.