In normal operation and especially in a fire or other casulity the closure of a valve must be positively verifiable and maintainable A valve that doesn't have positive shutoff can leak by and allow fuel to feed a fire. An example of a non positive valve would be an air operated diaphram valve (typically used as a regulator). With loss or reduction of control air pressure and spring closure of the stem fluid could still leak by if enough pressure is devloped under the disk to lift the stem. Another example of a non-positive closing valve is in a regulator where down and up stream pressure of the fluid is used to operate the valves internals. Merely backing off the adjustment spring will close the valve as long as there is adequate pressure up stream to maintain closure. However due to internal ports or loss of adequate upstream pressure fluid will be allowed to pass. EDIT: an even better example is a diaphram valve that uses an increase in air pressure to close. If there is a loss of air pressure or leak in the diaphram the valve mechanism will open. So in a fire the rubber diphram melts and the vlave opens wide allowing the free flow of fluid through the valve. A positive closing valve would be manual or motorized valve with a threaded stem which drives the disk against the seat no other force is utilized to position the valve disk against the seat. Because the disk is mechanically positioned with a screw thread it is impossible to raise the disc off the seat unless the screw threaded stem is turned in the opposite direction. Also there are not internal ports that provide a path through the valve Valves which are designed to make quick adjustments (regulate)to flow and pressure tend to be non-positive closing. While other common style valves such as Ball, Globe and Gate tend to be positive closing as the seating mechanism is operated by a direct mechanical means (Screw thread) and can be motorized to operated remotely.

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