IGST applies when a supply crosses state lines (an inter-state supply) or on imports into India. Instead of splitting into CGST and SGST, the full rate is charged as one IGST amount — an 18% inter-state supply is billed as 18% IGST.
The centre collects IGST and apportions the state’s share to the destination state, preserving GST’s destination-based design. Getting CGST/SGST versus IGST right depends on correctly determining the place of supply.