FAQ.md

@@ -29,6 +29,8 @@
     * create a group (sheet: group); assign all n units that "must run" to the group (uGroup); set constrainedOnlineTotalMax = -n (p_groupPolicy); for each unit (column *), set constrainedOnlineMultiplier=-1 (p_groupPolicy3D)
 * **I want to include a parameter in my input file that is not included in the example, so I don´t know where to put it.**
     * Search the backbone code for the parameter name and you will see from where it is read. You can for example search the backbone/inc folder using VS Code´s "find in folder" or search the Backbone.lst file from your last run using GAMS Studio.
+* **That p_gnu_io sheet doesn´t really make sense to me.**
+    * A unit can create (output), consume (input) or convert (in- and output) energy. The sheet specifies for each unit, from which node (in which grid) an input comes and to which node (in which grid) an output goes.
 
 # Pypsa-eur
 * **When using pypsa-eur, I get the error "symbolic privilege not held".**