I want to build a system where a document is to be signed/reviewed by the user. It may be the case that user takes 10 days to sign/review the document or even in minutes things are done.
By review : user shall review the document, but no need to sign it. Only accept/decline button or any alternative should be there so that we are aware of the outcome of the document.
By Sign : user shall go through the document and should sign it.
How to do review kind of flow?
The DocuSign system supports many different types of recipients. The most common one is called a Signer and that's the second one you mention. What you call "Review" has a couple of different names in DocuSign. In the web app it's called "Need to View" which is exactly what you describe, you must review the document, but there's no need to sign.
Using the eSignature API, this type of recipient is called "Certified Delivery" as DocuSign certified that the document was not only delivered, but opened and viewed (We can't ensure the person actually read it and comprehend it).
https://developers.docusign.com/docs/esign-rest-api/esign101/concepts/recipients/