Section 133(6) of income tax act, 1961

See before this power , when A.O ask banks to hand over some information , banks denies to deliver it stating that they are governed by banking act and its regulations and they cannot deliver the previlage information of its client .

so to provide a specific power section 133(6) had been introduce to provide the  power to income tax authorities to compel banks and banking instituions to furnish required information , which can be useful in any pending proceedings and inquire .

however as a control on misuse of this power it has been provided that where no proceding is pending ( i.e roving enquiry ) power can only be exercised by officer not below the rank of  CIT ( i.e CIT or DIRECTOR ) , or with the prior approval of CIT. 

Supreme court judgement  in case Kathiroor service co-operative bank ltd also based on use of this power 

section 133(6), gives the power to department to call for General  information from the assessee or even from the outsider for the purpose of income tax

Notice u/s 133(6) can be issue for cross verification of transaction. Suppose Mr. A enter into agreement with Mr. B. Normally when Mr A comes in to scrutiny and AO wants to verify the transaction of Mr. A with Mr. B then AO issues notice to Mr. B. By this way AO ensures identity of Mr. B and genuineness of the transaction.