Example from here on Meta.
Search for the term global - results from various categories will appear, including “bug” and “discourse hub”.
Search for global category:bug - relevant results are shown as expected.
Search for global category:discourse hub - no results appear.
Search for global category:'discourse hub' - no results appear.
Search for global category:"discourse hub" - 11 results appear.
Search for global category:discourse_hub - 11 results appear.
There is nothing in the search help to state double quotes or an underscore are required.