Found this bit of information buried in a larger document. I’ve found it’s valuable in upgrades. Feel free to ignore it if it doesn’t relate to you. I recently had a conversation with the tech doc folks for the protocol docs and they told me, Yeah. If you can help bring visibility to content that’s buried that you think is important… go for it. [Feel free to cut and paste and link back to us]. Here’s one of those cases.
In the MSDN article on “How to: Create a Custom Field Type”
“If you have a legacy custom field type developed under an earlier version of SharePoint, and it rendered differently on list views from the default rendering given to it by the XSLT rendering infrastructure, then you have the option to turn off XSLT rendering of the field. You do this by adding <Field Name="CAMLRendering">TRUE</Field> as a child of the FieldType element in the fldtypes*.xml file that contains the Collaborative Application Markup Language (CAML) definition of the legacy custom field. Using this option causes the field (and the column header) on list views to be rendered in accordance with RenderPatterns. For more information see RenderPattern Element (Field Types).”
Walkthrough: Creating a custom field type