Alternative means an option, a variant, a choice. Something from set of elements. So alternative is an element.
In the article there is example with JSON below.
value //set = value,
object // alternative from set value
array // and this one
string // same here
number // ...
"true" // ...
"false" // ...
"null" // ...
All types (bool,string, int whatever) are also a set and alternatives is all valid values.