I've seen materials' absorption coefficients listed with "A Mounting" specified. Read on Auralex FAQs that, "Type 'A' mounting means the test specimen was laid directly on the chamber floor," but then saw other info which confused me on the matter, such as a dealer whose treatment could be purchased "ready for Type A wall mounting."
Does Type A simply refer to direct mounting with no air space between wall and treatment?
Yes, Type A mounting implies it is placed directly against a boundary (the floor in the lab we've used).
While Type A does reflect a common mounting scenario for acoustic panels, the designation is mostly implied as a standard way to test so as to fairly compare test results against other products that may or may not have been tested at other labs.
Other common types you may see are Type E mounting (16" air gap between panel and boundary) and Type J (corner mounting).