we do these cutoff convergence like experimentally we add sample to obtain XRD peak..means hwo much sample is required to give peak,,,...so same like that we check the convergence of these parameters
you may find interesting to take a look at the tutorial "Siesta Caminando" authored by Sergei Apostnikov where some useful tricks for setting up your calculation within SIESTA are reported.
Anyway, one of the best feature of SIESTA is the possibility to make rather "good" calculations using the Default values for the parameters provided in the code itself. Of course for a "productive" use, you should converge the property in which you are interesting by varying smoothly each parameter.