Modifying Built-In SharePoint Files
Refers to any modification or deletion of the built-in files included with SharePoint Products and Technologies. This can include, but is not limited to, list schemas, Features, or site definitions.
Real World Example
A team wants a default list to behave differently, and wants to modify the file defining the list’s schema.
Except for a few well-defined files such as the web.config or docicon.xml files, you should never modify the built-in files included with SharePoint Products and Technologies except through official supported software updates, service packs, or product upgrades.
This type of customization is not supported.
Modifying the files that are installed by SharePoint is not supported. However, there are some scenarios in which you may have to modify these files to achieve consistent branding or other customizations. When you modify these files, you must consider that they may be replaced by future updates and service packs. Additionally, there may be complications when you upgrade to later versions of the product. Keep backup copies of all customized files in case they are overwritten by an update. Product support will provide commercially reasonable support for help with modifications but will be unable to provide product changes or hotfixes that result from modifying the files that are installed by SharePoint.
For best practices on customize the built-in files, see How to customize application pages in the Layouts folder in SharePoint Server 2007 and in Windows SharePoint Services 3.0.