Confused about ADT protocol

[Kevin <kevin@hackaday.com>]

GET and PUT seem to be making my head spin

put states 

Apple sends:
  One byte: "Z" ($DA)
  Byte stream: file name (null terminated)

Host sends:
  Two bytes: file size in blocks (LSB, MSB)
  One byte: return code:
    $00 = File exists (and file size is valid)
    $02 = File does not exist
    $04 = File exists, not recognized as a valid disk image

if the disk is coming from the apple how is the host going to find a file, measure its size and check for being valid? SO I am wondering if that could be backwards, what else might be?

Thanks