This feature is available from RMsis V.1.1.0 onwards.

Clicking on the cell corresponding to a requirement under the links column , opens up the following dialogue box

The user can click on a document or link to open the same in a different tab or window, as the case may be.

Otherwise, clicking on the Attach Document or Link, opens up the following dialogue box

The user can

enter the URL and press Save to attach specified link to the requirement.
click on "Browse", to open the following dialogue box in order to select a file (please note that this is dependent on your operating system) and click on upload after selecting the file to upload the document.

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