Not long ago, I have the same question about the definition of this term
and here's the answer:

Data Transfer Time - the total time it takes to send the total backup data
from TSM client to TSM server over the network.
      *** My understanding of this definition is :  client process will
read data from disk into I/O buffer, then from I/O buffer to TSM server
through
the network.  When TSM server receive these data, it will send a ACK packet
back to the client.  When the client receive the ACK packet from TSM
server, then
it will read the next segment of data from disk to I/O buffer.

===> Data Transfer Time is
the total sum of time that the client send the data to server through
network including the time it receive the ACK packet but exclude the time
to fetch the next batch of data into the I/O buffer.

Network data transfer rate - the rate that TSM client send the data over
the network to TSM server.

      *** My understanding of this definition is :  the total amount of
data send to TSM server divided by the data transfer time



Thanks & Regards
William