Instance annotations are rather similar to dynamic properties with a pre-known expressive name, i.e. the client can deduce from the name what type the dynamic property has.

For example: People?$filter=Age@numericValueException eq 'NaN'
would retrieve all people for which age is not a number.

If ODATA-735 gets accepted, clients can know up-front which instance annotations to expect, and can use them in filter conditions.

Open a separate issue (ODATA-97) to consider use in $select,/$expand.