in reply to your first Question :
The PSU is not giving any document to the employee concerned for he change of the agency. In fact these matters are confidential. Teh employees are physically and verbally been told about hte change of the consultancy by the Principle employer i.e. the PSU. And after that the documentation part is being handled by the PSU with the Manpower agency. Then a new joining letter is being issued to the employees wherein stating that the employee concenrned is deputed to the PSU in the designation of so and so............
Therefore in every joining letter it is written : the employees is being deputed to the PSU (name of athe Principal employer). this is one valid point to prove the continuity of the service despite of the breakage caused by the change in manpower agency.
In reply to your second question of resignation, i am not aware of that. But in the event of resignation it is certainly required to put paper to the Principle employer as well as the Manpower agency as the work station of the employee is the place or the office of the Peincipal employer / PSU. In the event of any sanction of leave, leave application is to be written to the Princpal employer / PSU.
Therefore on the above light of facts, when the employee is resigning, its the duty of the Principle employer to accept the resignation and release the employee in writing.