Advertise service area and focus area

[leonardr]

This is one of a series of proposals for taking NYPL's extensions to Authentication For OPDS and adding them to the core document.

https://github.com/NYPL-Simplified/Simplified/wiki/Authentication-For-OPDS-Extensions#service_area

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Some libraries and bookstores only serve certain geographical areas. The service_area object lets an OPDS server specify its service areas. This helps clients guide people to the OPDS servers that serve their area.

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https://github.com/NYPL-Simplified/Simplified/wiki/Authentication-For-OPDS-Extensions#focus_area

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A library's focus_area is the geographic area it focuses on, within its service_area. For public libraries, this is sometimes called the library's jurisdiction.

For example, the New York Public Library has a service_area of New York State, but its focus_area is limited to three of the boroughs of New York City:

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