I think the model is:
* We start with exactly two storage keys `container` (or a name that matches the API discussed earlier) and `sourceOrigin`
* Both keys are optional in all cases
* For `sourceOrigin` the default is the destination origin i.e. first party cookies.
* For `container` the default is the default container.
* In all cases both keys are accepted, but the command returns the parts of the partition key that were actually used, even if no value was supplied.