That 'Input-Gate' block at the far left side of the grid will have whatever signal(s) that are coming into Input(s) #1. What that signal is depends on how you have your 'Input #1' configured (LEFT ONLY/ L+R SUM/ STEREO) and what is plugged in to those inputs.
If set to 'Left Only', whatever is plugged into 'Input #1' left is mono, but is still present in both sides of the stereo signal path in that 'Input-Gate' block at the far left side of the grid.
If set to 'L+R Sum', whatever is plugged into 'Input #1' left and/or right is mono, but is still present in both sides of the stereo signal path in that 'Input-Gate' block at the far left side of the grid.
If set to 'Stereo', whatever is plugged into 'Input #1' left and right stays a stereo signal path in that 'Input-Gate' block at the far left side of the grid.
As you connect blocks to that 'Input-Gate' block, a left and right signal is present as it inters each block. Once again.....(What that signal is depends on how you have your 'Input #1' configured and what is plugged in to those inputs). Some blocks have a parameter that lets you select which part if the stereo signal it will allow into the block (LEFT ONLY/ L+R SUM/ STEREO). This allows you to send only the left or right side of the stereo signal down that particular signal chain to process separately. For example... A Piezo equipped guitar can plug the magnetic pickups into 'Input #1' left and the piezo output into 'Input #1' right and process them separately. Or two guitarists could share one Axe-Fx.
In my case, My 'Input #1' is configured for 'Left Only' (front). So
any block that is connected to the 'Input-Gate' block will have whatever is plugged into 'Input #1 left' present in both the left and right side of the stereo signal. This allowed me to process my guitar signal in 3 different ways and then re-combine them.
Sorry for the long-winded post..........There's still more to explain.....