The "Link on Social Media" option allows you to share a link to your design on social media. By clicking the link, anyone with access to the link can open your design in the PosterMyWall editor. Don't worry, they will open only a copy of your design (unless specified otherwise) and your original design will remain unchanged.
This option is useful if you'd like others to edit your design in the PosterMyWall editor. This can be used to collaborate with others in designing.
The "Publish to Social Media" option allows you to share the design, as it is, directly to social media. The image or video will be viewable and if it's a video, it can be played. If the published video or image was purchased prior to publishing, it will not contain a watermark.