The ability to define values for built-in metadata keys is explicit for certain content formats but implicit for others.
For example, when authoring in Markdown, an administrator would manually type the following line in a README.md file to set a document's content language to American English:
"ft:lang": "en-US"
In this case, defining a value for a built-in metadata key is explicit.
When authoring in a tool such as Author-it, however, the language configured in the tool's interface becomes the value of the ft:lang metadata key in Fluid Topics.
In this case, Fluid Topics works "behind the scenes" to transform information entered in one of the fields of an authoring tool's interface into a built-in metadata element.
Contrary to built-in metadata, defining custom metadata is always explicit and requires an administrator to define a metadata key and one or more values.