Final answer:
The precise prefix required for the FORMAT value of a "MetaData" key depends on the context and specific application.
Step-by-step explanation:
When the "MetaData" key is used in a structured data format, its FORMAT value must be prefixed by a particular string or identifier, which is often specific to the application or context in which the metadata is being used. However, as the question doesn't provide enough context regarding the particular structured data format (such as JSON, XML, etc.), programming language, or the system that it's referring to, it's impossible to provide an exact prefix without further information.
Typically, in structured data formats, a prefix may be a string literal that indicates the type of the metadata or a schema that needs to be followed. For example, in XML, it is not uncommon to see namespaces used as prefixes to distinguish between different sets of metadata. In JSON, keys don't require a prefix, but they need to be unique within an object, so a convention might be established to prefix specific types of keys to avoid naming collisions.
Overall, it's important to refer to the specific documentation associated with the metadata or the structured data format being used to determine the correct prefix for the FORMAT value of a "MetaData" key.