Can i consider that the parity bits remain unchanged during the transmission?
Alice sends a 7-bit string to Bob. At most one of those 7 bits can be flipped during transmission. We have no constraints on bits nor do we explicitly assign meaning (e.g. parity bits) to any of the bits.
As far as this project is concerned, the 7-bit message is just a binary string in \{0,1\}^7.