1a45265a_rigger_Rally_Map_XML_Reference__description
[attribute] middle [example value] 0.7 [possible values] a positive number under 1 [description] 
This defines at what brightness-level of the foliagemap a specific vegetation will be displayed. If you set it to a low value the vegetation will grow where the foliagemap is almost completely dark.
Trigger Rally Map XML Reference
description