[USR-TC] Firmware update question NMC and what is V.90 Enabled really mean?

I have a modem that has been V.90 Enabled, currently it is running version 5.4.95 I was thinking of upgrading it to 6.0.9 How will this effect V.90? What is "v.90 enabled NMC", is it hardware, software, firmware or some kind of key password challenge thing? Thank you for more info!