A "power latch relay" (called variously here a B+ relay or an ASD - Automatic ShutDown - Relay) is typically used in electronic engine management systems to provide power to a few systems after they key is turned off, so that they may return to "Engine Start" state from "Engine Run" state. It's meant to help decrease cranking times with start.
"Latching" a circuit, in electronics, is essentially "locking" it to a certain potential - briefly or permanently. "Latching to Power," in this case, means tying various circuit sources to battery power - it just happens to be for a definite time (30 seconds or so, I think.)
Is there any particular reason you're asking, or just for your own edification?
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