SAM (Stressed Assets Management) is a super-specialized outfit, wherein the SBI migrates high-value NPA accounts for initiating hard recovery action. This includes taking possession & auctioning the mortgaged assets under the SARFAESI Act, Filing recovery suits (OA) in DRT or invoking the jurisdiction of NCLT. Besides this the detective work, the forensic audit exercises are conducted to find out the incidences of fraud. The proceedings are initiated to declare the borrowers as non-cooperative / willful defaulters as per RBI guidelines. Besides this the complaints are filed with police authorities or CBI for investigation from the angle of fraud or misappropriation of stocks and other valuables by the NPA borrowers. 

When the default continues in repayment of installment or interest, for the time specified for the category of loan in question, the loan is to be classified by the institutional lender (Bank, NBFC or Financial Institution) as NPA Account.

In other words, an NPA is  a credit facility in respect of which the interest or installment of Bond finance principal has remained 'past due' for a period specified by the competent fiscal authority in any country. NPA is used by financial institutions that refer to loans that are in jeopardy of default to reflect its true financial positon in the Balance Sheet. 

In India, RBI is Banking Regulator and the NPA guidlines are provided in Prudential Norms Circular of RBI (Reserve Bank of India).

The Prudential Norms classify the loans for the purpose of NPA Classification in the two broad categories-
(A) Agricultural Loans:  For short duration crop agriculture loans such as paddy, Jowar, Bajra etc. if the loan (installment / interest) is not paid for 2 crop seasons , it would be termed as a NPA.

For long duration crop the above would be 1 Crop season from the due date.

(B) Non-Agricultural Loans:  If customers don’t repay either the interest or part of principal or both, the loan turns into NPA.

NPA in India is goverened by the Prudential NPA Norms of RBI.  For non-agricultural sectors an account becomes NPA if it remains irregular on account of non-service of interest or principal continuously for a period of 90 days.
On 8 November 2016, the Government of India announced the demonetisation, which caused imbalance in the business world leading to multifold increase NPA problem in India.
The GST was implemented in India with effect from July 1, 2017.   The scheme further intensified the issues in business transactions and cuased liquidity problem leading to more business failures.  To give relief to the businesses the RBI relaxed the NPA classification announced 180 days NPA norms vide notifiation dated 07-02-2018.

AUCA means advances under collection account.  This is the written Off Loans portfolio of a Bank.  AUCA is an off balancesheet head.  Any recovery in AUCA is profit for a Bank.

Interest is not applied to NPA Accounts because the recovery of principal in in jeopardy in NPA Accounts and if a Bank books interest by applying the interest in its ledger system, this will artificially increase the taxable income of the Bank.  But, this does not relieves the customer from his liability to pay interest on the debit balances in NPA accounts.  The interest is charged at the time of calculating final dues in NPA accounts.