The opposite of generalization is specialization, and it is associated with type augmentation in F#. It is worth noting that the official F# 4.0 Language Specification (http://fsharp.org/specs/language-spec/4.0/FSharpSpec-4.0-latest.pdf) does not introduce this terminology using type extension instead. Nevertheless, the type augmentation expression is de-facto ubiquitous and used interchangeably with type extension. Personally, I believe that augmentation is free of the undesired connotation that extension carries as something that's added to an existing matter. Augmentation is a better synonym for the specialization of an existing matter by adding, customizing, or even removing features. So we will stick to it here.
The following figure shows two flavors of type augmentation available in F#. Both use the same syntax but represent different use cases. Intrinsic augmentation customizes your own code, while optional augmentation may customize types outside of your code: