This read-write object selects the BERT pattern to be sent and expected to be received, and the description notes that in some implementations it cannot be modified once the test is running. Because BERT works by sending a known bit sequence and measuring how many bits come back wrong, the pattern choice matters: different patterns are more or less effective at exposing particular kinds of line problems, such as clock recovery issues, so picking the wrong pattern for the fault being chased can make a real problem look clean. Running a simple all-ones pattern might pass cleanly on a line with a marginal clock-recovery circuit that would show heavy errors under a pseudo-random pattern designed specifically to stress that function.