section 143(1) covers the intimation being sent by the dept to the assessee within the period of one yr from the end of financial year in which return under section 139(1) or in reponse to notice under section 142(1) is being filed by the assessee. the intimation is sent because while assessing total income or loss there existed any arithmetical error or an incorrect claim has been made.


143(2) is the notice being served by the dept to the assessee for security or regular assessment. but for serving this notice it is a neccisiate that the assessee has filed the return u/s 139(1) or in response to notice u/s 142(1. thus, if the AO considers it necessary to ensure that the assessee has not understated the income or have computed excessive losses or has not underpaid the tax in any manner.

143(3) is the security assessment.

The Scrutiny Assessments under Income Tax Act 1961 are made u/s 143(3). For many years now many of the returns of the assesses are accepted as they are being filed by the assesses and intimation is sent u/s 143(1) and only fewer cases are selected for scrutiny assessment based upon some predetermined criterias. Therefore every assessee desires that his return should be accepted as it is filed u/s 143(1) and not subjected to scrutiny. Some important points relating to the Assessment and scrutiny Assessment are discussed herein below:
When a case is selected for scrutiny: Where a return has been filed u/s 139 or in response to a notice u/s 142(1), the case can be selected for scrutiny Assessment if the assessing officer considers it necessary or expedient to ensure that:
- the assessee has not understated the income or has not computed excessive loss, or
-has not underpaid the tax in any manner.

Usually the amount of turnover, rate of gross profit, total income, quantum of loans taken, investments during the year etc are considered for selecting a case for scrutiny. However the selection is done on the basis of the instructions of the CBDT issued every year wherein some predetermined criteria’s are decided on the basis of which a case is selected for scrutiny Assessment.
And for making assessment u/s 143(3) notice under section 143(2) has to be issued within time prescibed to call the assesee to furnish the desired information and seek answers to various questions by the ITO.
If no notice u/w 143(2) is received within the prescribed time by the assessee, then no Assessment can be framed u/s 143(3).

refer from Income tax india.gov.in