Easily create a link to another post using a shortcode to reference the post by id or slug; the link text is the post's title, unless overridden.
The "Enable legacy HTML comment-style tag support?" setting controls whether the older, HTML comment style notation (from before v2.0 of the plugin) should be recognized. The "Enable legacy pseudo-shortcode tag support?" setting controls whether the older pseudo-shortcode syntax (from before v3.0 of the plugin) should be recognized You should only enable a particular legacy mode if you've used versions of this plugin prior to v3.0 and still want those post-to-post links to work without updating them. If you started using the plugin at v3.0 or later, you do not need to enable legacy support. Unnecessarily enabling legacy tag support will have a negative impact on the performance of your site.
If no post on your blog matches the value you specified as either a post ID or post slug, then the post-to-post tag in question disappears from the view, having been replaced with an empty string.
Yes. See the Filters section for an example of code to change the shortcode name. If you are going to make this short of change, it is recommended that you do so before making use of the shortcode in your posts/pages (unless you search and replace all occurrences of the original shortcode in other posts). Out of the box, the plugin only supports a single name for the shortcode.