DHO,
The spec you refer to is an alloy heat-treated, artificially aged, and cold worked to improve its strength and corrosion resistant properties.
The mill as a general rule supplies the material at the T6 temper with the other tempers being done by the customer.
If you can get the mill to do this for you, which they will with some sheets and plates, it will save you time and money if you do not have to do any forming on the part.
here is a related thread thread330-336927
B.E.
You are judged not by what you know, but by what you can do.