From the description of "what's new" in B139: Requirements have been introduced for auxiliary tanks operating in a continuous overflow condition to reduce the risk of over-pressurization or vacuum collapse of the tank
Think of it as having your pot sitting in the sink, being filled from the faucet above. If you keep filling, it will eventually run over the lip. This would be considered an example of a "continuous overflow" condition. Now apply that thought process to whatever it is you're looking at (presumably some kind of storage element, rather than a piping run).
Converting energy to motion for more than half a century
