Section 8.1.2 of http://hl7.org/fhir/uv/sdc/2018Sep/populate.html mentions the use of the "variable" extension (http://hl7.org/fhir/2018Sep/extension-variable.html) which defines an expression to be used in getting the value for the variable, but I don't see any place that allows the name of the variable to be chosen. Maybe it is somewhere else, but if so the documentation needs some clarification.