Modern doors are always about 1/2" narrower than their listed size. There has to be some gap around the door, and that 1/2" is your gap. The INSIDE of the frame is the listed size (32" 34" 36", etc).
If you're replacing an entire door and frame, you need the OUTSIDE measurements of the frame, an you make your rough opening about 3/4" larger, so you can align the frame and apply the shims.
Some OLD doors were the actual listed size, but not always.... (Of course back then, the frames had to be 1/2" bigger than the listed door size).