Leftfield is the catch-all for downtempo that refuses a lane, held together by taste rather than by tempo or style.
It's the selector's genre: Four Tet next to a library record next to a dub plate, unified by a digger's ear and a love of low end. And at its most cerebral it becomes something harder to name: melody and emotion wrapped around programming intricate enough to reward close listening. That end of the spectrum, the Aphex Twin and Boards of Canada end, came out of the post-rave comedown and Warp's Artificial Intelligence series, but it lives here rather than in its own bin.
The mix series (DJ-Kicks, Late Night Tales, Fabric's downtempo entries) are its natural home, curation as an art form.
If a record is too eclectic, too dubby, too curious, or too brainy to file anywhere else, this is where it lives.