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This field should be populated with the key of the value to be inserted. An example could be “JPEG:Compression Type = Baseline“, where “JPEG:Compression Type“ is the key, and “Baseline“ the value. Multiple keys, separated by , can be specified. In this case, the first key for which a value can be extracted is used. Thus, multiple keys can be used to provide fallbacks.
These key-value pairs are found using this tool
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click on the settings icon: 
→ System administration → METADATA → Asset info → click on 
and add new meta field
Fields to fill , out: Name, Field type and upload tag name - as shown below:
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