Every .vl document has at least one patch, the document patch which gives an overview of the content in the document. The document patch of the active document can always be reached via the shortcut Alt+P.
The most common type of patch is a datatype patch. It roughly corresponds to what other programming languages call a "class". Every datatype patch has a corresponding type-definition in the document patch.
In both cases the corresponding type-definition is automatically placed in the document patch of the active document.