SWITCH case for string types

In HELP ABOUT_SWITCH, the second example shows that the case literal, if it
is a string, does not have to be inclosed by "".

However, in the complete syntax section that follows, it says:

"string" | number|variable|{expression} { Statementlist }
default { statement list }

What does "string" in the sytax section mean? I tried it out and it appears
that string does not have to be surrounded by "s. Which leads to a bigger
question: how to differentiate between the string "default" and the default
case?

Thanks.