mustSupport is expected to be the name of an element, or a path to an element on the data type, and it indicates that the knowledge module will use that element as part of it's evaluation. So, for example on a Patient DataRequirement, mustSupport of "gender" would indicate that the knowledge module uses gender to perform it's calculations.
For the various path elements, the same approach is taken, it's just the name of the element, or a path to an element. For paths, use dot invocation to traverse through an element.
There is a nuance here that we are still working through, whether the filter paths should use a search parameter name, or should be expressed in terms of the elements of the data type. Right now, the examples in the spec use the names of elements of the data type, which means that implementations have to determine how those names map to search parameters in order to provide indexed access. We're actively seeking implementation feedback on that aspect.
I hope this helps