You are basically using an available vPin as a placeholder for the Bridge. Each Bridge define needs its own vPin, and that vPin is not used for anything else.
So you simply define a source Bridge name to a vPin and call that Bridge by said name AKA bridge1 to control the desired vPin of the targets device/code.
Thus…
Uses the source device/code virtual pin V21 to “assign/store/whatever” the bridge named bridge1
while…
This command for bridge1 in the source device/code sends the data in t_buffer1 to the target device/code virtual pin, also named V21 - But, the correlation between vPin numbers is irrelevant and they can be totally different from each other as they are referring to seperate devices.