I understand it simply to mean that the tank will have to stand the deformations occurring at its base at the ultimate hypothesis without failure, i.e., you use such soil modulus (applying it to get spring constants for elementes at the base of the tank) for the FEM analysis you will be performing to attest the tank is valid both in deformation as in strength.
Normally, the deformation analysis would be service level, for such is the ordinary interpretation, but if in doubt, contact who better knows. If he doubts, make him notice that a structure in its normal use is NEVER to surpass the service level loadings; if it does, then the safety factors that are precisely there to such purpose of ultimate strength enter in action, but is a matter of exceptionality, not normalcy.