It is not clear the relationship between Salad documents and document contexts

The [section 2.3 in the spec](https://www.commonwl.org/v1.2/SchemaSalad.html#Document_context) describes two kinds of document contexts: implied context and explicit context.
It seems that the Salad documents are written in either implied context or explicit context.

However, the [metaschema of Salad Schema](https://github.com/common-workflow-language/schema_salad/blob/e90a5574519c29b6887a962ec4d408ccb00eb526/schema_salad/metaschema/metaschema.yml) imports [`field_name.yml`](https://github.com/common-workflow-language/schema_salad/blob/e90a5574519c29b6887a962ec4d408ccb00eb526/schema_salad/metaschema/field_name.yml) that is not written in implied context nor explicit context.
Also, the [import section](https://www.commonwl.org/v1.2/SchemaSalad.html#Import) says that the imported URI must be loaded as a Salad document.

Can a document that is not in implicit context nor explicit context be a valid Salad document?
If so, where or when can we use document contexts in a Salad document?