HTML elements can have 2 classes:
h1 and h2 both have the class "book". h1 has 2 classes assigned: "book" & "domestic". h2 has 2 classes assigned "book" & "foreign"
When you set a style for the .book it will change both h1 and h2 since they both have that class.
When you set a style for domestic it will apply that style to only h1.
Same goes for if you set a style for foreign it will only apply to h2 in the example.