Recently, I was asked to explain that different museums use different controlled vocabularies and metadata schemas. Most links are to the Society of American Archivist's dictionary. This is an introduction to the frustrating work of cataloging. Each type of institution uses a controlled vocabulary in their required metadata fields. To be super confusing different types of institutions use different metadata schemas. This means that the required metadata fields (filled using a controlled vocabulary dependent which controlled vocabulary is used) varies often dependent on the type of museum/archive/etc... For example, art museums may use the Art and Architecture Thesaurus while libraries may use the Library of Congress subject headings as their controlled vocabulary. It is a big deal to change a word in a controlled vocabulary, and there are groups that are working to change some of the more problematic words in various controlled vocabularies. This made sense as institutions can choose a metadata schema and a controlled vocabulary to meet their institutional requirements and best serve their audiences. The various museums/archives/etc.. each uses a metadata schema (which controls the required metadata fields) and a controlled vocabulary (controls which words go in the metadata field) that best serve their audience. Many of these decisions were made a long time ago. But it is helpful as it allows those institutions using the Art and Architecture Thesaurus to share records and researchers of art history learn what fields to expect in a catalog record and what should be in those fields. Also, it allows the digitized materials at a single institution to easily make their database searchable. In the case of the main Smithsonian Institutions Catalog Search page, it is searching a variety of metadata schemas (chosen by the institution a long time ago) and controlled vocabulary also chosen a long time ago. Slowly, in the digital age Crosswalks are slowly being developed to map the different metadata schema to each other. Here are two examples of materials from the Smithsonian Institution catalog search using different metadata schemas and controlled vocabulary found using the term "morter" a misspelling of mortar.
http://collections.si.edu/search/detail/edanmdm:fbr_item_MODSI4159
https://collections.si.edu/search/results.htm?q=morter
Another example of a metadata schema is DACS which is what I used when creating the finding aid for the menu collection I did not use a controlled vocabulary.
Usually, the metadata schema and controlled vocabulary come as a set.