Bonnet is part of valve that enclosed a packing box and seal which guide the stem during movement . Bonnet could be of standard or extented type.
The later type is used when extreme process temperature (high or low) becomes a concern since it can break the stem packing.
Extension is needed to bring the process temperature move toward the ambient temperature before reaching the packing box.
In term of connection type to valve body, bonnet could be bolted, threaded, or welded type.