If the Mass Summary shows Story 1 with only 600 kN, you may have forgotten finishes or a transfer slab. If Story 5 shows 1200 kN (equal to lower floors), your model likely has an incorrect load pattern on the roof—unrealistic for most structures.
The coordinates representing the "centroid" of the mass for that specific story. This is used by ETABS to calculate accidental eccentricity and torsional effects. etabs mass summary by story
If ETABS reports total mass = 100,000 N·s²/m, then $W = 100,000 \times 9.81 = 981,000 N = 981 kN$. This must match the sum of dead load plus applicable live load per ASCE 7. If the Mass Summary shows Story 1 with