S-Docs allows you to generate and attach new documents to your current email right from the email page in Salesforce. This is useful if you forgot about certain documents you wanted to send but don't want to exit the email page and start all over. All you have to do is add the  GenAtEmail='true' parameter to your S-Docs button.
You will then see a Generate New Documents button appear on the email page whenever you click the S-Docs button that you edited. You can click it to select more documents to generate and automatically attach to your current email.
Note that the S-Docs button comes prepackaged for certain standard objects, such as the Contact object in the example above, and that this button cannot be edited. If you wish to enable the Generate New Documents on Email Page feature for these objects, you will need to create a new custom S-Docs button and add it to your page layout, and remove the prepackaged button from the page layout (if it's already there). Visit the Quick Install & Configuration Guide to learn about how to create an S-Docs button.