Use ` to define literals

CONSTANTS some_constant TYPE string VALUE `ABC`.
DATA(some_string) = `ABC`.  " --> TYPE string

Refrain from using ', as it adds a superfluous type conversion and confuses the reader whether he’s dealing with a CHAR or STRING:

" anti-pattern
DATA some_string TYPE string.
some_string = 'ABC'.

| is generally okay, but cannot be used for CONSTANTS and adds needless overhead when specifying a fixed value:

" anti-pattern
DATA(some_string) = |ABC|.