Few comments/questions after reviewing the document:
1). Is the first parameter of the STRING test (list of variable names)
has to be "expanded"?
2). ABNF (or RFC 3028 "Syntax:" line) for MODIFIER is missing.
Also, I have an editorial suggestion. Section 5. ("Action set") doesn't
mention modifiers at all, instead there is a subsection 5.1 for them
I would suggest to add the following sentence to section 5:
Modifiers are applied on a value before it is stored in the variable.
Modifier names are case insensitive. For more information see section 5.1
And drop the first 2 sentences at the top of 5.1.