Salary structure for Expat assignments in MNCs

I'm curious about the salary structures that MNCs follow or Expat assignments given to employees hired in India that excel and get a chance to work in offices outside of India. There's very little clarity on how specifics are determined, at least that's the case in our company. If possible, try to cover these aspects in your answer:

1. Duration of these expat assignments: Is it few months/few years/permanent?
2. Salary of India office: If the expat assignment is temporary, is it the norm that over and above your expat salary, you'll continue receiving your India salary tax free?
3. Once the expat assignment is completed, are you expected to come back to India office and give KT to others, or you'll continue doing more expat assignments?
4. What perks/allowances do you generally get as a part of the expat assignment, which you did not get in the India office?

Please mention any other relevant aspects or weird incidents that you might have encountered first hand or of someone else going on an Expat assignment.