Section 122 of Income Tax Ordinance 2001 ( PAKISTAN INCOME TAX LAW)

Amendment of Assessment

The Commissioner may amend an assessment order treated as issued U/S 120 ( Return submitted deemed to be assessed) U/S 121 (Best judgement assessment) by making such alteration or addition as consider necessary.
The Commissioner cannot issue order after expiry of five years from the end of financial year
in which the Commissioner has issued or treated to have issued assessment order.
When a taxpayer furnishes a revised return U/S 114 ( 6 A)
The taxpayers revised return shall be taken to be amendment assessment on the day on which the revised return was furnished.
The Commissioner can amend assessment as many times as may be necessary. This mean that Commissioner can issue notice U/S 122 for a year more than one time.
The Commissioner shall issue assessment order if he consider that
- any income   chargeable to tax  has been escaped
- total income has been under stated
- excessive refund has been claimed
 The Commissioner shall issue an amended assessment order to the taxpayer stating
01. The amended taxable income of the taxpayers
02- the amended tax amount due
03- the amount of tax paid ,if any
04- the time, manner and place of appealing the amended assessment

122 A Revision by Commissioner

The Commissioner shall not revise any order if 

01. an appeal against the order lies to the Commissioner or to the Appellate Tribunal ,the time within which such appeal may be filed and time is not barred.
02 the order is pending in appeal before the Commissioner appeal or appeal has been made to appellate tribunal.

122 B Revision by Chief Commissioner

The Commissioner either on his own will or on application filed by taxpayer for revision call for record of any proceedings relating to issuance of an exemption or lower rate certificate for deduction / collection of tax.
The Chief Commissioner make such order after inquiry and providing an opportunity of being heard to taxpayers which deem fit in the circumstances of the case.