This CCK field allows a filtering of the selectable terms for a matching vocabulary displayed in the taxonomy select combo box in a node create/edit form. This allows a single vocabulary to be used across a number of node types. It is used to restrict the terms associated with a node type. Subsets can be overlapping. A single selection will force that term to be associated with the node type and the selection will not be displayed. Typically the term will be the node type's name.
This approach allows easy filtering using the vocabulary with the Views module.