This option will apply the Glass Type selected in the list to all instances of this element in the project (universal change). Even if this element is used on Surfaces on different orientations to the one selected, all will update.
If this is not the intention, here is a second option to assign the new Glass Type to the selected Surface only. See Section 126.96.36.199.2 below for this option.
To control Glass Types on different area of the project, new window or curtain panel types may be created in Revit under a family name. This may be an option to help manage performance/specifications on different Surfaces.