Why use BANDWIDTH_SR for ud_bw(rather than ud_bi_bw)?

For ud_bw test, the client side just send and the server side just receive, so why use BANDWIDTH_SR rather than BANDWIDTH? Intuitively speaking, BANDWIDTH_SR should be used for bidirectional bandwidth, however in the current code, BANDWIDTH_SR is used for all unreliable trans modes tests, not bidirectional bandwidth tests.