The Nifty Bank Index holds the most liquid and bluechip stocks. The index has large Indian banking stocks. Investors and market intermediaries get a benchmark that controls the capital market performance of Indian banks. Entry and exit of bank stocks in nifty is depends on market capitalization of stocks other macro econmic factors.

Nifty bank index involves a maximum of 12 companies listed on the National Stock Exchange of India (NSE).
As on July 31 2019, below is the Bank Nifty;
HDFC Bank Ltd.
ICICI Bank Ltd
Kotak Mahindra Bank Ltd
Axis Bank Ltd.
State Bank of India
IndusInd Bank Ltd.
Federal Bank Ltd.
Yes Bank Ltd.
RBL Bank Ltd.