AVCHD Index File
Advanced Video Coding High Definition is a format created for recording and playback in high definition. The format was created by Panasonic and Sony primarily for use in Camcorders. Files encoded using AVCHD can be stored on DVD, Hard drive, memory card or internal memory in the device.
On the storage for the AVCHD videos you will find a file with the .bdm file extension this is an index file and contains information regarding the video.