These are both document placeholders within the Documents section of the Side App. Anything in the Active Documents section will be sent for signing when the Prepare Envelope button is selected.
The Inactive Documents section is simply a holding area for documents that are not ready to be sent for signing.
You can move the documents to Inactive Documents by: Clicking the 3 dots on the document tile (card view) or to the right of the document source (list view). Click on [Move to Inactive]
The documents that are moved to inactive can be moved to "active" if needed