a6 is if you want to file for an EAD before filing for AOS, while the i-94 is still valid. Nobody does this as the i-94 is only valid for 90 days and it takes much longer than that for an EAD to get approved. So completely useless, in other words.
The category you use is c9, if you file together with the i-485. That is the code you write on Part 2, question 27 on form i-765.
You find all this information in the i-765 instructions, I suggest you read them, as well as the instructions for every single form you plan on submitting.
