The nominal MLC position can be set to either the light field edge or the radiation field edge when defining the RFO.
The Eclipse treatment planning system requires the nominal position of the MLC to be defined by the light field, and then the algorithm decreases the MLC position values set for the actual modeled radiation field by the constant RFO parameter in order to align the nominal MLC position (defined by the light field) with the modeled radiation field.
Other systems may require the nominal MLC position to be defined by the radiation field to start with, noting the importance of treatment planning specific physical commissioning of the MLC.