On one side a thermal sensor is associated with one thermal zone. On the other side a governor manages one thermal zone. The governor acts on a cooling device.
Some configurations have multiple sensors for the same performance domain. For instance, one sensor per core and a DVFS per group of cores.
When the energy model is used along with the power allocator, having a shared cooling device for all the thermal zones makes very difficult to have a the PID loop to work correctly as all governors can do opposite actions.
The proposal is to aggregate the sensors belonging to the same performance domain into a single thermal zone. Consequently one governor can operate on the temperature returned by the an aggregator function (eg. max or avg).
As a side note, the device tree bindings already describes such a an aggregation but it seems it was never implemented.