This both configures how many test cells to send in a loopback diagnostic session and doubles as the trigger to start that session -- writing any nonzero value here when it was previously zero kicks off the test, and the value then counts down as cells are sent. An admin running a sustained loopback test to catch intermittent cell loss would set this to a large number like 10000 to get a statistically meaningful sample. For example, writing cktOamLoopbackCount to 500 starts a session that sends 500 loopback cells, after which the admin checks cktOamLoopbackReceived and cktOamLoopbackTimeouts to see how many round trips actually succeeded.