Eight pins (pins 34 to 41) are prepared in the center of the BT121 package, and the pin-out description in the datasheet says "RESERVED FOR PRODUCTION PROGRAMMING".


This pin is provided by Silicon Labs for use in the manufacturing process and does not require soldering.
The data sheet "Figure 23 BT121 module recommended PCB pad pattern" describes the following, but please pay attention to placement of vias and traces to prevent short circuits.

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The 8 center pins of the module are reserved for production programming and should be left unsoldered but despite of this unmasked traces and vias should NOT be placed under the module in order to prevent short circuits.

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