A BDM file isn’t governed by a single format and in video usage it often means Blu-ray/AVCHD BDMV navigation files—INDEX.BDMV or MOVIEOBJ.BDMV—that describe structure rather than store video, while actual streams are .m2ts/.mts under BDMV\STREAM and playback logic is defined by .mpls playlists and .clpi clip info, which explains why BDM files don’t open as videos; in backup contexts a .BDM may be a metadata index describing what was backed up and how large files are split or verified, usable only with its original backup program, and in other cases apps or games pack internal resources into .BDM archives readable only by their own tools.
The easiest way to identify a BDM file is to inspect its origin and neighbors, because the meaning changes by system: if it came from camera media or a disc-like folder, it likely belongs to BDMV/AVCHD where BDM/BDMV files store structure rather than video, especially if you see STREAM, PLAYLIST, CLIPINF, or .m2ts/.mpls/.clpi files; if the BDM file sits next to giant data chunks, it’s typically a small backup index, whereas if it’s located inside a game/application folder it usually holds proprietary resources for that program.
"BDM isn’t a single universal standard" shows that .BDM can represent unrelated structures since software creators can assign the same three letters to totally different file types, making a BDM from one workflow unrelated to a BDM from another; that’s why BDM could be disc-style navigation metadata, a backup catalog, or an internal data container, and the only reliable method to classify it is context—source folder, companion files, and size—not a one-size-fits-all viewer.
A BDM/BDMV-related file tends to show up in workflows that author or record content like Blu-ray/AVCHD, so it normally lives inside a BDMV directory alongside STREAM, PLAYLIST, and CLIPINF subfolders; in that arrangement the BDM/BDMV files act as metadata while .MTS/.M2TS files in STREAM store the real footage, and the same structure appears in Blu-ray disc copies or authoring program exports—so anything that looks like a disc export will include these files inside or next to a BDMV folder rather than providing a single video you can open directly.
The fastest way to confirm a BDM file is to look at companion folders, since the same extension can mean different things: a BDMV folder containing STREAM, PLAYLIST, and CLIPINF means Blu-ray/AVCHD metadata with real video in .m2ts/.mts streams; a tiny BDM next to large split files points to a backup catalog; and a BDM mixed into program/game install files suggests application-specific data—so the quick rule is disc-style folders = Blu-ray/AVCHD, small-plus-large pattern = backup, everything else = app/game.