Before the world moved to using PCs as the main interface to talk to the 400, we had IBM Keyboards with keys that are now largely disappeared from the keyboards of today.
Now people need to remember to use the universal F1 help key.
Hex here does not mean Witchcraft but Hexadecimal which is IBM name for Base 16 that Midrange computers used to run on (it might be a higher one now). This means that while humans count from 1 to 10, the 400 counts from 1 to 15 where the single character representing 10-15 is A-F.
Each byte is represented by a pair of hexadecimal characters, such as
- letter B upper case represented by blank then B
- letter b lower case represented by 8 then 2
Since 16 x 16 combinations comes to more than all the letters digits and special characters we are familiar with on most keyboards, there are a bunch more characters made available, which got used for such things as currency symbols around the world, and characters in some languages with a marking on top to give it an accent.
There were some intermediate keyboards where we had to use ALT then some key to get the HEX.
So to get at some character not on our keyboard, we would press the HEX key then the two characters in sequence that mapped that hexadecimal representation. One of the RPG manuals has a reference chart in an appendix showing all these characters.