You cannot author the document by viewing the page on Web UI, or drag-and-drop, or assign signatory roles using transientDocumentId through API.
The transientDocuments call returns the transientDocumentID, which is valid for 7 days. You can use it only for the further API calls. It is stored on the API server and assigned this ID. Upload a file and then refer its ID in the further API calls.
You cannot directly upload a document in Agreement using the REST API.
As per the REST requirement, first create a transient document first and then use this ID in Agreement, Widget, or Library methods as the file source.
The transient document is a raw source file such as PDF, doc, docx uploaded to the Adobe servers. So, it is a convenient way to transmit your base document to the API servers and a transient document on the web.