Understood. So, it is likely an issue with the Flex settings. It seems to agree with other observations that I made: With my 746Pro, I've been able to connect with stations with weaker signals than the Flex station.

N5ARC is a Flex 6600. NO6GN is an IC-746Pro. The issue is that when NO6GN attempts to connect to N5ARC, the replies from N5ARC can be audibly heard immediately after each connection attempt Tx from NO6GN completes (VARA is configured to make 10 attempts), but VarAC doesn't catch the replies and the connection attempt fails. However, when N5ARC attempts to connect to NO6GN there is no issue with VarAC (on NO6GN) hearing N5ARC this time and the connection is successful. Connection attempts between the two stations are successful in only one direction where the Flex sends connection requests to the IC-746Pro. Do you think that this is an issue with the configuration settings on the Flex?

Missed the settings screenshots.

Band conditions decreased a bit at the time these screenshots were captured, but the behavior was the same. I tried to capture these within the briefest time possible. N5ARC was sending replies at the time NO6GN was attempting to ping.

I worked with another station and it looks like the problem was that I was in an "I'm away" state at the time of the request. No issue after all.

It is checked.

Based on the user manual for IC-746Pro, the concatenated commands I copied and pasted from IC-746 for USB-D seem to be correct. The rig seems to work properly with the software now.

Perfect! I will play with these and reply here if I find the command that works.

How does one know what string of letters and numbers to put into the mode commands?

It does, but now it only stays in USB-D regardless of what mode is selected in the software settings. This is fine for me right now since I am only interested in data mode with this software. For Icom IC-746 Pro (CAT command version 7.2), I modified "ModeUSB_D=FEFE66E00604FD" to "ModeUSB_D=FEFE66E00601FD,FEFE66E01A0601FD". I copied it from Icom IC-746 just above it without really knowing what I was doing or understanding the command codes. I just noticed that in the revision history a change was made for IC-746 around USB_D (v6.2).