Power Query allows information about a value to be attached to a value.

Ordinarily, these descriptive tidbits are invisible. Their presence does not change the mashup engine’s behavior. However, code (yours or others’, such as code in the standard library or in the host application) can intentionally choose to read and set these annotations, known as metadata, using them as a way to communicate extra information relevant to a particular value. These details can simply be for informational or diagnostic purposes; they can also be factored in by expressions as they determine how to behave.

Take parameters as an example. Parameters get special treatment in Microsoft Power BI’s user interface, yet in Power Query itself, they are stored as ordinary values.

This is good. You shouldn’t need to do anything special to consume a parameter; it should just be a value to your code. Yet parameters are special, but where is that “specialness” (like their configuration settings) hiding? From Power Query’s perspective, the answer is—yes, you guessed it—in metadata.

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