https://retrocomputing.stackexchange.com/questions/3045/what-was-the-purpose-and-history-of-the-c64s-special-keys

"The Commodore "C=" Key

The VIC-20 removed the numeric keypad that the PET keyboards had, combining the numeric and punctuation keys on the top row with the unshifted keystrokes giving numbers and the shifted keystrokes giving punctuation (!, ", etc.). They also added colour and assigned character codes to change the colour of the text. A good guess would be that this is the reason they added the "C=": it's a second kind of shift that now allows three PETSCII codes to be produced from each key rather than just two. This allows all the original graphics codes still to be produced and adds enough extra keystroke inputs to cover the new colours as well. The same keyboard and decoding was used on the C64, with a few extra color codes added.

Thus, while SHIFT L produced PETSCII code 204 on both the PET and the C64, SHIFT 6 produced code 182 on the PET but an ampersand & on the C64, and to get that code 182 on the C64 you'd instead use "C= L"."