I am using the 'set approval status' action in my workflow to mark a document as approved or rejected. Which variable is set by this action?

it updates sharepoint's internal document status flag.

see documentation for the action - http://help.nintex.com/en-US/nintex2013/help/#Workflow/RootCategory/Actions/Nintex.Workflow.SetApprovalStatus.htm%3FTocP… 

Hi Marian

Thanks for the help. But I want to retrieve this in-built status flag value and display it to the user by updating a column in the Documents list. 

you can directly show 'Approval Status' field in library view.

if you really want to get status within workflow, there is as well 'Approval Status' Item property reference.

Oh that's great!

But I had made my custom 'Approval Status' column which I think has now overwritten the inbuilt column. Is there any way to revert back? I am unable to get back the original one.

what happens if you simply remove your custom column?

default one should be automatically brough in from content type.

note you will not see it among library column in library settings, but you should see it in view configuration.

You will have needed to enable Content Approval in the library settings for the Approval Status column to appear I believe.

Enable this setting under Versioning Settings and then you should be able to see the column in your library and workflow.