For calculations under ASCE 7, CII User Guide mentions now
W+T1+P1+0.7U1 as operating case with seismic load for computing stress (which is as ASD method, Allowable Stress Design, that compares actual and allowable stress) and
W+T1+P1+U1 as operating case with seismic load for computing loads on supports and deflections (based on this case you can isolate seismic loads transmitted to structure for a LFRD calculation of your structural assembly (Loads and Resistance Factor Design, which provides the actual response of structural system to be compared with ultimate strength design).
So U or 0.7U (as horizontal seismic) and other combinations with vertical- it depends on your applicable Code (is ASCE 7 - which consider a MRI 2475 years as seismic definition/ or maybe other Code) and your scope (want to make an ASD analysis on your piping or want to collaborate with structural for their LFRD analysis).