The standard meter consists of one, two or three current transformers that output accumulated power on one display.
The bidirectional meter is useful with solar and wind powered systems connected to the grid. The meter senses the current flowing in each direction and displays it on a separate display counter.
The 2-in-1 Denali meter allows a single meter to measure two separate 120 volt services. It is most often used in marinas where a single power pedestal is shared by two boats in adjacent slips.
The 2-in-1 Olympic meter is a special use, three element meter that displays the power measured by one CT on one display and the power measured by the other two CTs on the other display. The Plus or Plus IO version of this meter is also necessary if you want your computer to be able to record the power used on each of the three phases.