"A utility is the jargon for any radio station on the short wave band that is not for the public's entertainment or hobby. All those huge gaps on your shortwave radio, the ones between broadcast and amateur bands, are utility bands allocated by international treaties.

The word "utility" comes more or less from the original Latin for "usefulness." With very few exceptions, these radio communications are most certainly being used as part of some mission or task, known or unknown.

Utilities are harder to hear than broadcasts, but they're way more fun. You'll find all the world's militaries, ships, aircraft, spies, bootleggers, embassies, and anyone else who needs to communicate over the horizon. While satellites are often primary, the failure of Galaxy IV shows once again why many people want a shortwave backup. If your radio has a "USB" or "CW" setting, you're ready to give utilities a try.

The Utility Planet column and its web site are intended as friendly, jargon-free clearinghouses for utility information, so you'll hear more signals and fewer odd noises. In other words, you folks write both of them."

https://ominous-valve.com/uteworld.html#utedef

On Wikipedia

https://en.wikipedia.org/wiki/Utility_station

Digital Modes Samples

https://web.archive.org/web/20181117011715/http://kb9ukd.com/digital/

https://wiki.radioreference.com/index.php/RadioReference_UDXF_Portal

NOTICE: The rules about listening to signals not intended for you applies here. The contents of many signals might be considered sensitive by the party sending and the reception of such signals may be illegal in your country. The authors neither condone or encourage such acts

https://www.hfunderground.com/board/index.php?board=2.0

We seek to understand and document all radio transmissions, legal and otherwise, as part of the radio listening hobby. We do not encourage any radio operations contrary to regulations. Always consult with the appropriate authorities if you have questions concerning what is permissible in your locale.

Why Numbers Stations?

https://radiohobbyist.org/numbersracket/numbers/NUMBERS/NUMBERS.HTM

Priyom.org is an international group of radio enthusiasts who came together during 2010 when activity from the station known as "UVB-76" increased. The mystery of this station inspired us to seek out other similarly mysterious stations, and thus into the bizarre world of shortwave radio's infamous number stations.

https://priyom.org/

https://www.numbersoddities.nl/