There are tens of thousands of fonts in our Web Fonts library, so listing all of them can be a little too much to take. So, when listing the fonts in our library, you can narrow the results based on values specified for certain filters..
To get the available filters, use "List Filters".
To get the possible values for a filter, use "List Filter Values". You provide the filter whose values you desire, along with a values for any other filters you provide. For example, if the filter of interest is "Designer" and you provide Language=Hebrew and Classification=Handwriting, then you will receive a list of those designers who have handwriting Hebrew fonts in our library.
Of course, you might not want to make this call for every filter, so you can use "List All Filter Values". For this API, you don't specify any filter, since all possible values for all filters will be returned - given the provided value of any filters that you pass in. In question form, consider this API "I'll provide what filters the user has currently selected, and you return to me the possible values for all filters".
This is useful for populating drop-down lists for the font-browsing experience.