It comes down to how much the fabric is protected when the awning is retracted. The A1 Awning is an open system — the rolled fabric is exposed, though an optional hood can be fitted to cover it. The A3 Awning has a semi-cassette that shields the fabric from above. The A2 Awning has a fully closed cassette with an additional front-bar gasket, sealing the fabric away from the weather completely when closed. More enclosure generally means better long-term fabric protection and a neater facade.