See Graham's comment.
Rant warning!
In my opinion, the term "quick part" is an abomination and whoever at MS came up with it should be fired. It is a recipe for confusion.
Custom Document Properties are not "quick parts."
"Quick Part" is:
- A button on the Insert Menu.
- A building blocks gallery, the contents of which will appear on that menu and get in the way of other controls there.
- AutoText is included on that menu. It is now one type of Building Block.
- The Building Block Organizer is included on that menu and gives control over "quick parts" as well as other building blocks.
- Mislabelled Document Property Content Controls are on that menu. These include only a small subset of Document Properties and they are Mapped to the XML. They are very useful but never include custom document properties. Repeating Data Using Document Properties Content Controls and Other Mapped Content Controls
- Fields are listed on the menu because MS couldn't find another place to put them. (They could have simply put another button on the Insert tab.) They have nothing to do with the rest of "quick parts." There are the AutoText, and DocProperty fields which do access those quick parts.
Sorry for the rant. As you can see, I do not work for Microsoft.