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# Key_namespace

A 32-bit identifier describing the domain and the structure of the key value.

## Overview

For example, this could indicate that `key_value` is a reverse-address style string (such as “com.apple.quicktime.ISO6709”), a binary four-character code (such as a `'cprt'` user data key), a Uniform Resource Identifier (URI), or other structures (such as native formats from other metadata standards). New key namespaces must be registered but because a reverse-address style string can often be used, using the reverse-address key namespace may be sufficient for most uses.

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