This optional attribute indicates whether this link type may be deleted by users. There are 4 required types:
This list may vary in subsequent versions.
The default value, also the case if this attribute is omitted, is false.
It is to be assumed users can set existing or user-added types as 'required'. Users should not attempt to alter the 'required' state where this has been set by the program defaults (the above list). This does mean that if you use custom versions of the file, when upgrading the program, you should review the version of the file in the application bundle for any changes to use of this argument and adapt your custom file accordingly.
<link name="prototype" color="#993333" required="true" />