As Zepplock says, that’s actually two classes in a single attribute: boolean and optional. The space is not part of a class name; it acts as the separator.
These three selectors will all match it:
.boolean
.optional
.boolean.optional
The last selector only picks up this element as it has both classes.
You never include a space when chaining class selectors, not even like this:
.boolean .optional
As that selects .optional elements that are contained within separate .boolean elements.