Hello Lokesh,
For the bank audits, you have to empanelment yourself or your company to the rbi for statutory audits and for the concurrent audit, you have to empanelment yourself with specific bank.
Mostly audit is given by merit basis, you have more partners and more qualification, chances of audit increases. So the DISA is very compulsory course required for bank audit, because every bank calls such firms which partners have disa qualification. Other factors like experience and no. of partners are also considered.
So this is not a certain or specific proess. but every bank gives his minimum requirement for the empanelment which you have to fulfill.. Some times for the banks of small cities, you will get the audit easily if you have an branch office in that city..