What is source and sink?

In plain language

A Blu-ray player, console, streaming box or PC is a source. A TV, projector or monitor is a sink. Simple enough — until you put an AV receiver in the middle, at which point it is a sink to the player and a source to the TV, running two separate negotiations at once.

Why it matters in your setup

Thinking in source-and-sink pairs is what makes a complicated chain tractable. A five-device system is not one HDMI connection, it is four, each with its own EDID exchange, its own handshake and its own bandwidth ceiling. A fault lives in one of those links, not in “the system”.

It also explains why a device can be blameless and still be the problem. A receiver that reports its own capabilities rather than passing the projector's through is behaving exactly as designed — it just means the source is being told about the receiver, not about the screen.

What the DR HDMI 8K does with it

The DR HDMI 8K is deliberately both: a sink to your source and a source to whatever comes next. That is what lets it stand in the middle and control the exchange, and why its status page separates everything into TX (towards the display) and RX (towards the source).