Closeoff is how much differential pressure the valve / actuator combination can overcome in order to seat the valve and stop flow completely.  The valve you're looking at is meant for small differential pressures -- a circulating system for heat or cooling.  Its spring (or motor) can only apply so much force against the water pressure difference across itself.
If you have typical domestic pressure (100 PSI maybe) and you're wanting to shut off in case of a major leak (near zero pressure), you'll have almost the full system pressure to close off against.
Since you're in the Johnson Controls book, look at their ball valves.  You should be able to find a non-characterized, full-port ball valve in the appropriate size that will handle 100PSI plus.

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