| Database abstraction layer | includes/database.inc | Allow the use of different database servers using the same code base. |
| Extra blogapi updates for 5.x | modules/blogapi/blogapi.install | |
| File interface | includes/file.inc | Common file handling functions. |
| Form generation | includes/form.inc | Functions to enable the processing and display of HTML forms. |
| Formatting | includes/common.inc | Functions to format numbers, strings, dates, etc. |
| Hook implementations for user-created content types. | modules/node/node.module | |
| Hooks | includes/module.inc | Allow modules to interact with the Drupal core. |
| Input validation | includes/common.inc | Functions to validate user input. |
| Menu system | includes/menu.inc | Define the navigation menus, and route page requests to code based on URLs. |
| Node access rights | modules/node/node.module | The node access system determines who can do what to which nodes. |
| Search interface | modules/search/search.module | The Drupal search interface manages a global search mechanism. |
| System updates from 4.7 to 5.0 | modules/system/system.install | |
| System updates from 5.0 to x.x | modules/system/system.install | @todo Start this series of updates at 2000. |
| Themeable functions | includes/theme.inc | Functions that display HTML, and which can be customized by themes. |
