This argument was proposed by Jake Nebel, an assistant professor of philosophy at University of Southern California.
Brightline (sometimes called precision) is a measure of how clear the division is between topical and non-topical cases under a certain interpretation.
An Education standard asserts that the negative's interpretation of the resolution focuses the debate down to the most important area(s) for learning.
An example might be a case under a topic about limiting the use or stockpiling of weapons of mass destruction that declares war on North Korea or Iran.
The Negative team would argue that such a case would only possibly be topical if it could be proven beyond a doubt not only that Iran or North Korea had weapons of mass destruction but also if such a war did not increase global proliferation pressures or involve the eventual use of weapons of mass destruction or did not lead to looting of such stockpiles, all very tendentious assumptions.
Negative teams will typically argue that such plans drastically abuse the resolution (i.e. allow too wide of a variety of cases to be run).
For example, a plan under an energy-conservation topic might both sign the Kyoto Protocol and increase general science funding across the board, obviously including energy conservation.
A Negative team would argue that this would be extra-topical because the plan is acting in areas that are outside the boundaries of the resolution (therefore, "extra"-topicality).
Affirmative teams will either argue that extra-topicality is legitimate or, much more frequently, that all components of their plan are in fact topical.