Swift variable name with ` (backtick)

According to the Swift documentation :

To use a reserved word as an identifier, put a backtick (`)before and after it. For example, class is not a valid identifier, but `class` is valid. The backticks are not considered part of the identifier; `x` and x have the same meaning.

In your example, default is a Swift reserved keyword, that’s why backticks are needed.