Is it required ti get relieving letter if moved from Parent company to its subsidiary?

Hi,
Please clear my Doubt.
Two companies being run by the same management. Lets us consider one as X and the other as Y. I am have been working with company X for the past 4 yrs and was asked to move to company Y. I have received a mail stating that I have been moved to the other company - Y with the same roles and responsibilities. After being moved, the name of the company Y is changed to Z.
Is it required that I get a relieving mail from company X?
How should I showcase my work experience in my resume?
Regards,
Nagarajan Sekar

Speak to management if they are interested to provide you experience/relieve letter then better keep that Letter for your future records.

Please retain the mail, it indirectly acknowledges your 4 years of service before you are moved to new one, will be useful to claim gratuity after completing even one year of service.
Please ask company to issue you new appointment letter as you will be working under new legal entity. Check for clause on transfer of service. If it not there, mildly suggest them to incorporate the same. (Even if they do not incorporate the clause, you have the mail which mentions that you are transferred to new entity. But then you will have to fight for gratuity of 4 years as & when you separate from the company.)