Storage for Arduino Boards Manager JSON and package files for the STM32 and STM8 cores support.
For more information see the Wiki. Mainly the Getting Started page.
If you have any issue to download/use a package, you could file an issue on Github.
Or submit a topic on the stm32duino forum:
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questions on the STM32 Core
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bugs/enhancements on the STM core: Bugs and enhancements