ION8650 socket meters have a volts mode for 29S and 39S. When looking at the wiring diagrams in the installation guide they look identical.
ION8650 socket meters
On socket meters, the volts mode modifies the reference spades for each measured quantity.
IE which quantity, V1, V2, V3, I1, etc... is connected to which input terminal of the meter.
The picture below shows the difference between each socket and the corresponding volts mode. This is all dependant upon physical connection and the base socket installed.
Please note the information in the picture notes that a 9S base can function as any of these 3 volt modes depending upon your physical connections to the base.