I haven't yet played with the new "WALL_SKINS_PARAMS" global parameter. But from what I read, the global is made up of an array totalling 12 columns and 1-8 rows. The columns indicate the referenced parameter. The rows store the value of the skin parameter (up to 8 skins).
Arrays are distinguished in GDL by: PARAMETER NAME[row#][column#].
For example - the 1st row of the 5th column would be: PARAMETER NAME.