In Two Accounts ^web extensions are used to inject JavaScript in html. You are enabled to use it for custom url or everywhere. These extension are useful for web developer. It is like js console to run JavaScript. Eg: if you want to change a button or field font color or background color or drop down selection etc you are able to do it using JavaScript. To create extension go to Settings > PDF | Themes | Extensions > Extensions. Click New Extension then enter name in name field then add JavaScript then set url Custom or Everywhere in location field. Custom url is like "sales-invoices", Then click Create button. Administrator users are enabled to create or edit or delete extensions. You may edit or inactive or delete it later.