They are used to evaluate the data transfer rate or the number of operations in a unit of time. 1 T/s means one transfer per second, 1 MT/s means one million transfers per second, and one GT/s means one billion.
There is a direct connection between the transfer rate and the data transfer rate via the bus width. Since the transfer rate only tells how many transfers take place in a unit of time, but does not take into account the number of bits transferred in parallel on a bus, the data transfer rate can be determined by multiplying the transfer rate and the bus width. For example, if the transfer rate is 2 GT/s and the bus width is 8 bits, then the effective data transfer rate is about 15 Gbit/s. Related to the parallel transfer of one byte each, this results in a byte-related data transfer rate of2 GB/s.