IS 1893 (Part 1):2016 divides India into four seismic zones, numbered II to V, and gives each a zone factor Z: 0.10 for Zone II, 0.16 for Zone III, 0.24 for Zone IV and 0.36 for Zone V. Those numbers are not arbitrary weights. Z is the peak ground acceleration, as a fraction of g, for the maximum considered earthquake at that location.
The part that gets copied without thought is the division. The design horizontal acceleration uses Z/2, not Z. The reason is that Z describes the maximum considered earthquake — broadly a 475-year return-period event — while ordinary buildings are designed for the design basis earthquake, which is expected to damage the structure without collapsing it. Halving Z is the code stating that trade explicitly rather than hiding it in a load factor.
The importance factor sits on top. Table 8 runs from 1.0 to 1.5: ordinary residential and commercial buildings take 1.0, buildings with large occupancy such as schools and offices take 1.2, and hospitals, fire stations and emergency facilities take 1.5. The consequence is concrete: a hospital in Zone IV is designed for one and a half times the base shear of a house on the same plot.
Two things this does not do. It does not tell you the building will be undamaged — the design basis earthquake anticipates damage. And it does not substitute for ductile detailing: the response reduction factor that divides the base shear is only earned if the detailing that justifies it is actually built, which is a site problem more often than a design one.
If you take one number away, take Zone III at 0.16 against Zone V at 0.36. That is a factor of 2.25 on ground acceleration between Bhopal and Guwahati, decided by a line on a map that many drawings never state.