What are the default DSP filters?

Hi all, I am working with a buddy of mine putting together a hardware serial device to perform CAT commands on the Flex. I am a little confused what the:
ZZFI Reads / Sets VFO A DSP Filter Index
Command does. The documentation is a little sparse on this one. I understand it reports on the bandwidth but what exactally do the values mean? Also how does this relate to the low and high cut RX filters that can be set with another command. Any explanations are greatly appreciated.