Medium- and High-Rate Packets
In addition to single-slot, three-slot and multislot packets, Figure 2 shows medium-rate (DM) and high-rate (DH) packets. Medium-rate packets carry extra error protection: The information and CRC bits are coded with a 2/3-rate FEC that adds 5 parity bits to every 10-bit segment. In a noisy environment, the DM packets can be a good choice, even though the overhead of the FEC requires you to pack less data into each packet.
Users don't have to choose packet types because Bluetooth Link Managers can automatically choose packet types to fit the error rate in the environment.