This plugin allows you to insert a user specific text (such as copyright, credit, advertisement, etc.) at the beginning/ending of your posts.
In release 2.1.0, the "Add Header and/or Footer Text For Old Posts" option will always be enabled when you re-install/re-activate the plugin. The reason is to fix the issue in release 2.0.x series. In release 2.0.x, this option is not defined because of WordPress behavior changes for plugin updating.
Here are some detail information for "Add Header and/or Footer Text For Old Posts" option:
In release 2.0 and above, I add a new feature: allow user decide which posts display the header and/or footer text. This will add a Custom Field, named "posturl_add_url", for each new created post. But for the posts you published before use WP-PostURL 2.0+, they do not have Custom Field "posturl_add_url". For those posts, you need configure "Add Header and/or Footer Text For Old Posts" option. If you check this option, all existed posts will be added the header and/or footer text; otherwise those posts will not be added the header text and/or footer text. If some of those posts are against with this default behavior, you need to re-edit this post to set "Add Post URL?" option for those special posts.