What is SRCREV_machine and SRCREV_meta in yocto kernel recipes

Question

Why doesn't the kernel recipe simply use SRCREV="${AUTOINC}" or hash. I find these lines instead.

https://github.com/agherzan/meta-raspberrypi/blob/master/recipes-kernel/linux/linux-raspberrypi_6.1.bb

SRCREV_machine = "31dbf25138831241f31f7eee835b83a607eaa179"
SRCREV_meta = "2eaed50911009f9ddbc74460093e17b22ef7daa0"

What do these define? How does bitbake process these?