Locating a feature to its self is incorrect. Though you could control the location of its edges controlling the size (i.e. profile) of the feature to its centerline. Have not seen this in my 20 years. The closest thing to this is defining a slot as centerline giving it a size and locating one edge of slot to centerline (dimension being on half of slot size). This allows you to shift the centerline to one side or the other as needed giving a bonus tolerance to sizes and mainly locations of other features on part. Not a good practice but has been used a lot in the past before cad cam and the wide use of true positions and the use of profiles to define the size and form of a part (caused lots of debates between supplier and customer).
The first call-out with datumes controll the size, shape and form to the datums. The secound one controls a tighter shap and form to its self no location.
The first one would be like a vertual contition (size, shape and location). The secound one the form and size of feature with no location
One use of would to allow a male to fit a female tight but allowing it to float with-in the larger profile.