The 3.3V REG board is used to reduce the power supply voltage. The additonal board should be provided with a 5V DC voltage via a screw terminal CN1.
This voltage is reduced and stabilized by using the on-board voltage regulator LP2950. The reduced voltage amounts to 3.3V and is delivered from the additional board via a screw terminal CN2.
Project Page is here: 3V3 Reg Board