Rich
--- a/docs/customize/extensions.md
+++ b/docs/customize/extensions.md
+If you positioned your extensions directory outside of the docroot, you'll need to put in place some special server configuration to make extensions' public assets accessible by http.
+
+
+## Advanced: Other Extension Locations
+
+CiviCRM will find, use and (via the web interface) manage extensions in the url and path location defined above, the "default" extension location for your install.
+
+But CiviCRM will also find and use extensions in other locations. These locations might be populated by other mechanisms - e.g. a vetted subset of all extensions that are made available by default by a hosting provider. Specifically, CiviCRM will scan three directories and their subdirectories for info.xml files, similar to the mechanism that Drupal uses to automatically detect modules. The three directories correspond to three extension 'containers', as defined in the function "getFullContainer" of the file "CRM/Extension/System.php"
+
+1. default : the one identified the settings. But note that although CiviCRM will only manage the extensions in the immediate directory location, it will still find them in subdirectories.
+1. civiroot : the civicrm module code root, i.e. where the civicrm-version.php file is. This allows CiviCRM to be distributed with extensions (e.g. SearchKit), and different 'distributions' of CiviCRM to be created.
+1. cmsvendor : a subdirectory 'vendor' of the CMS root (typically, the docroot of the web service). Analagous to the (core) module directory of the CMS. This one is not available for WordPress, only Drupal and Joomla, as noted in the code.
+
+## Advanced: Using unstable (alpha/beta/pre-releases etc.) through the Manage Extensions screens.
+
+You can change the _extension repository URL_. This is the URL used by CiviCRM to fetch the list of available extensions, and it can be customized by editing "civicrm.settings.php":