Systems should not be required to do both. A system should not try to simulate a facility if the simulation has side effects. For example, if simulating the capability of moving the byte pointer to the desired position has some side effects, then the simulation should be left to the process accessing the file. All implementors should make known the capabilities of their implementations via NIC documents. The FAP extension to FTP includes 6 new commands and the file pointer.
Any implementation requires the file pointer and all six commands.
But, as described above, it is not necessary to implement the commands in their full generality. The file pointer represents an index or address within the file. In particular, for transmission mode Stream and structure Record, the EOR and EOF markers are not counted. Local transformations on data must be taken into account. If transmission Mode is Image then the logical byte size is taken as the transmission byte size.