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.