• a constant (for example 3 or 42.0)

• a numerical index field (for example, my_int_field)

• a ranking key defined in the query, prefixed with @ (for example, @my_ranking_key)

• a call to a built-in function, which can have arguments that are valid virtual field expressions

• an n-ary numerical operator (n=1,2,3) applied to n virtual field expressions