S-Drive Email allows you to send email with attached S-Drive files from any record, not just the record you’re sending from. S-Drive Email is used within the S-Drive component when files are selected and the email envelope icon is clicked. However S-Drive Email can also be a standalone component, as described here.
S-Drive Email supports email threading when using Email-to-Case. Replies to email sent will show in the Case activity history.
To add S-Drive Email to your Lightning page
Edit the Lightning page you want to add S-Drive Email to.
Choose S-Drive Email Lightning component and drag it onto the page.
Save the page.
The standalone component gives the option to attach S-Drive files from any record, use email templates and use email templates and quick texts.
See 2.20 User guide for Salesforce Lightning | Emailing-Files for more on using S-Drive Email.