Hi, we've found the solution, i'll try to sum up what we've learned from what you guys have told me and our tests in the system.
The generation of the document it's the result of the real time integrarion between CO and FI, as Paulo said since the new G/L it's active any post generated in CO will create an FI document, even if in theory that post is between two CO elements. So there is no way to avoid it from appearing.
In the real time integration FI/CO configuration (spro) you can set the Document type used for every post generated in CO.
With this i can solve the original problem wich was to diferentiate the FI documents caused by the settlement from any other FI posting.
Thank you both for your help in this matter.
Pelayo.-