Answer:
There is an option to call up this URL while RFEM is running:
This shows the definition of the entire API as XML (see also WSDL https://en.wikipedia.org/wiki/Web_Services_Description_Language).
A pragmatic way to determine the parameters is, for example, to first compile the desired material in RFEM, and then read out the properties. The following program shows the procedure:
from RFEM.initModel import *
Model(True, 'Material.rf6')
Model.clientModel.service.begin_modification()
m = Model.clientModel.service.get_material(1)
print(m)
Model.clientModel.service.finish_modification()
Model.clientModel.service.close_connection()
This method can be used for all objects in RFEM.