What are the meanings of the beep codes in the developer screen?

I am trying to boot a custom image from usb. I have run enable_dev_usb_boot as root, but upon pressing ctrl+U (or ctrl+L) in the developer screen (with the flash drive inserted), it beeps and doesn't boot.

For ctrl+U, it's a low, ~0.3s beep

For ctrl+L, two high ~0.1s beeps with a ~0.1s interval.

What do they mean?