USD13 FrSky Sbus to C-PPM decoder. these are used on most every F3/F4 board when all the UART are USED for GPS/Compass/Telemetry/OSD/BLE and allows you to use C-PPM -> SBus Rx such as L9R - for example iNav as PPM can be a soft-serial connection whereas Sbus requires a dedicated Serial Port, and when you have a lot of NAV serial devices - it's back to PPM for RC.
the jumper pin changes the PWM spacing from 21ms Default with no jumper) to 28ms. The 28ms option is intended for systems which require Ch 1~12 ; the default 21ms option outputs ch1~8 (not all boards/software can handle 28ms!! Check Google results for your board first) - DEFAULT = 21ms/8Ch