This sets how many errors (that each cause a temporary relay closing) are allowed on a single Tau port after phantom comes up before that port's relay is closed and the port detached; it's re-opened once phantom goes down and back up. Admins tune it to control how tolerant the system is of a marginal connection before giving up on it entirely. For example, lowering this threshold isolates a flaky station's port from the ring faster, at the cost of being more sensitive to occasional transient errors.