Gets or sets the category of building blocks which shall be used as the filter when determining the possible choices of building blocks which are displayed for insertion into the content control. A building block category is a sub-classification within a given building block gallery which can be used to further categorize the parts in a given gallery. If value is null, then the content control shall display all building blocks in the specified gallery regardless their specified category. This property has effect for content controls with type BuildingBlockGallery and BuildingBlock.