On the left side of the rule, the items to the left of the + are characters, whereas the item to the right of + is a keystroke, which can be expressed either as [K_XXXX] or a character which is interpreted as a key code.
And yes, @lorna is correct. rule is made up of three parts:
context + keystroke > output
The context consists of characters already in the document, so a keystroke makes no logical sense in that part of the rule. (Note, while the idea of emitting a keystroke to the active application is not illogical, we don’t support that either )