When creating custom content types inside your features or site definitions you need to specify field definitions to define your custom fields. I was trying to create a Enhanced Rich Text field within my Content Type but was struggling to find the right XML.

Eventually I found out you need to include the RichText="TRUE" and RichTextMode="FullHtml" attributes within your field definition as shown below:

<Field

ID="{8A2EA652-DDCB-4bf7-84EE-129E7B7A5403}"
Name="ArticleBody"
Group="Reuters columns"
DisplayName="Article Body"
Type="Note"
Sealed="FALSE"
ReadOnly="FALSE"
Hidden="FALSE"
RichText="TRUE"
RichTextMode="FullHtml"
DisplaceOnUpgrade="TRUE" />

I can't understand why this information isn't more readily available and well documented on MSDN.