It's the Carret Notation: https://en.wikipedia.org/wiki/Caret_notation
35 karma · joined May 14, 2017
History and Architectures
Edited by Raúl Rojas and Ulf Hashagen
On Error Goto line
"The cart data is stored using a steganographic process. Each PICO-8 byte is stored as the two least significant bits of each of the four color channels"
But they are logical (architectural) registers and are mapped at run time to one of the physical registers.
https://pygame-zero.readthedocs.io
https://github.com/mjbrusso/game2dboard
Disclosure: I am the author of game2dboard