In plain language
Linear Pulse Code Modulation is audio with nothing done to it: the samples, as they are. It is described by three numbers — channel count (2.0, 5.1, 7.1), sample rate (up to 192kHz) and bit width (up to 24-bit).
Because it needs no decoder, every HDMI device supports it. That makes it the format a chain falls back to when anything else fails to negotiate.
Why it matters in your setup
You choose between letting the source decode to LPCM or sending the compressed stream on for the receiver to decode. Both are legitimate. LPCM is more robust; bitstream keeps the receiver's own processing and its format display in play.
One caveat: object-based formats cannot survive the trip as LPCM. Decode Atmos to PCM at the source and the object metadata is gone, so the receiver gets a channel mix instead.
What the DR HDMI 8K does with it
L-PCM is the first audio flag in the DR HDMI 8K's EDID controls, with its own settings for channel count, sample rate up to 192kHz and bit width up to 24-bit. The EDID readouts show the PCM capability on both sides, so you can see at a glance whether the source is being told 7.1 is available or only stereo.
Need the hardware?
Open the DR HDMI 8K product page on HDfury.uk.