Conditions are used in any case where a branch, or a boolean value is required (as explicitly marked in a function).

is %Item%

Determines whether this item would result in a "true" expansion as an RValue. The item need not exist to use in an is clause.

exists %Item%

Determines whether this item reference refers to an existing item in the context. This returns true if the item exists, even if that item is null.

Condition.Function ...

Calls one of the appropriately marked Condition functions and uses the return of that function as the result of the condition.