The ASAVR output format can send either 20 MHz or 12 MHz MLat according to that switch.
Bit packing tells the hardware to pack every 2 x 12bit samples into 3 bytes instead of sending every samples as a 16bit integer. This saves 25% of the USB bandwidth, but leaves less headroom in the MCU for synchronizing the USB with the ADC. In general, it helps with some old computers or SBCs.