For tds deposit ? to distinguish between psu & govt. co.

As the Govt. deductors has to deposit on the same day , and others by 7th of the following month

I want to know how to distinguish PSU and Govt. co.

For example banks are considered PSUs  and the companies like NMDC, National Textiles are considered Govt.

I want the legal provisions to convince myself as well as my clients .

(here 1 may note some govt. offices pay through book adjustment some companies through challan)