sec 153(1) and sec 143

Notice u/s.143 (2) can be served within the period of 6 months from the end of the financial year in which ROI is filed. Please read the section carefully.

 oops i made a mistake while typing question.i have corrected the question now.

If you read both the sections carefully, there is no complexity at all. Sec153 (1) prescribes time limit for completion of assessment u/s.143 or 144.  Assessment u/s.143 (3) by issuing notice u/s.143 (2) is a comprehensive scrutiny. It requires the assessee to produce any evidence which the assessee may rely in support of the return filed. Assessment u/s.144 is Best judgment Assessment (i.e. Ex-party assessment).

 yes. i agree but what is the purpose of assessment order?

The assessing officers assess the income as declared in ROI. He verifies the correctness of income declared, tax paid thereon etc. He ensure that the assessee has not understated the income or not computed excessive loss or has not under-paid the tax in any manner. After completing verification in comprehensive scrutiny, he passes the assessment order of his findings and determines the total income or loss and sum payable by the assessee on the basis of such assessment order.