TDS SECTION 51

Sir recently TDS for government contractor have been started I am working with a office a single Executive Engineer in Chandigarh who is giving services in Chandigarh , Punjab, Himachal. Say Head office is in Chandigarh or paying authority in Chandigarh. Now the contractors who are giving services in Himachal Pradesh have the GST No of HP and the executive engineer has only one No. GST No for the tax deduction of Chandigarh Then how the contractors working at HP will show the sale as to registered dealer or unregistered dealer and how the TDS will be deducted by Ex-En as there version is that department will deduct the GST as IGST and deposit to same to Govt. And also PL guide either the contractor working in all three states under one office has to get three GST No separately or only to one state and If I have the GST no of Chandigarh and can I work at Kerla with the same GST No if I get the contract from there Thanks