Independent and Asynchronous means the same thing, i.e. they are independent outputs.
This means that the outputs can be configured separately from the video grid.
Updated 20/1/22
Model Name HDMI/VGA Output
NVR302-E2-P HDMI and VGA independent outputs provided
NVR302-E2 HDMI and VGA independent outputs provided
NVR304-16X
HDMI1、 HDMI2、VGA asynchronous output（HDMI audio output is not supported）
NVR304-E2-P HDMI and VGA independent outputs provided
NVR304-E2 HDMI and VGA independent outputs provided
NVR304-32X
HDMI1、 HDMI2、VGA asynchronous output（HDMI audio output is not supported）
NVR308-16X
HDMI1、 HDMI2、VGA asynchronous output（HDMI audio output is not supported）
NVR308-32X
HDMI1、 HDMI2、VGA asynchronous output（HDMI audio output is not supported）
NVR308-64X
HDMI1、 HDMI2、VGA asynchronous output（HDMI audio output is not supported）
NVR516-64
VGA、HDMI1、HDMI2 asynchronous output（HDMI audio output is not supported）
NVR516-128
VGA、HDMI1、HDMI2 asynchronous output（HDMI audio output is not supported）
NVR824-128R VGA、HDMI1、HDMI2 asynchronous output
NVR824-256R VGA、HDMI1、HDMI2 asynchronous output