By using the Transfer Size option, the client can determine how much data is transferred on a read request (RRQ).
This is useful for requesting a subnet broadcast of a file. The client finds the size of the buffer it needs in order to store the file in memory. Drawing from this block size, the client determines the number of blocks for the transfer. The number of blocks is helpful information for tracking the blocks that have been received. You can also use it for the last block acknowledgment (ACK), which must be sent to end a transfer normally. Without the Transfer Size option, determining the size and the last block of the transfer requires the client to wait for a block to be received that is smaller than the block size of the transfer.
For files transferred in netascii mode, this option might not be as useful if you are converting the data during the transfer in a way that changes its size. Also, the system might require additional processing time to determine the transfer size due to conversion of the file to the appropriate coded character set identifier (CCSID).