If the software does not provide a stiffness value in the anticipated direction, the assumption is to use maximum resistance. Thermal expansion of the unit attaching the two vessel sections does not occur without resistance.

In-plane and out-plane does not make sense to me in this scenario as the CL of the nozzle is parallel to the CL of the vessel section. As Lukachiri demonstrates, it only makes sense when the two vectors are non-parallel, and preferably perpendicular.

You'll need to review your software documentation to be certain, but noting that one value is 4x the resistance of the other direction, you can intuit that it's easier to twist the nozzle in the horizontal plane rather than the vertical plane due to its proximity to the edge of the vessel.