This is the default option. It is the combination of an unknown bound and an open bound. The limits of distributions that are non-asymptotic are treated as in the unknown bound case, while asymptotic distributions are still included as in the open bound case.
Note, not all distributions functions are compatible with all the possible choices. For example, you can not specify a fixed or unknown lower bound for the Normal distribution, since it asymptotically extends to minus infinity.
