The Special Variable allows for Text, Phone Number or Email type variables once you start typing text in the Value 1 property. However, it only allows for alphanumeric characters in the value properties, which makes it impossible to enter an email address for example.
In my case, I need values like "Before+" (not allowed) and "Before-" (allowed), while they would fit perfectly in the assigned special variable, which is a Text.
(Edit: This is in version 8. I didn't confirm yet that the problem exists in version 9 as well)