The flat-circular stick on is designed for mounting your sensor. You can peel and press on. Once the tape cures (12 hours), it provides a solid base. This mounting base comes in two thread sizes, M3 and M4. Use the M3 base for MIPS 50, 75, 100, & 150 sensors with a heim joint. If you are using this with the snap joint post, use the M4 variant.