The square-wave sent by the EVSE to the car is a continuous function - you can signal any value you like between 6A and 80A with no steps as it's an analogue value.I think it is a step function if the EVSE detects resistance in a certain band it selects the appropriate current by sending a mark-space ratio square wave to the car charger. The choices would be 32A, 16A, 10A, 7A I think. If the EVSE and lead are both 32A then the mark-space will be for 32A but the car charger will only draw 16A if it is a 3.3 car.
The car itself isn't obliged to follow the signal precisely, merely to keep its total consumption below the value signalled by the EVSE (you could imagine a very simple design at the car end with two 6A chargers - one of them turned on all the time, the other turned on only when the EVSE signalled 12A or more). In practice, most cars seem to be more flexible than that and offer either continuously variable control or fairly fine steps (I don't have detailed info for the Leaf).
The resistors used for cable identification by the EVSE do have a limited number of steps specified in the standards - probably too crude to be very useful for solar charging, but in theory available on any socketed EVSE.
The resistors used by Mainpine on their controllers are a proprietary design feature by Mainpine so they can in principle do anything they like in their software, though apparently limited to a modest number of steps in current versions.