As per Section 2(40) of Income Tax Act, 1961, unless the context otherwise requires, the term “regular assessment” means the assessment made under sub-section (3) of section 143 or section 144.
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Assessment u/s 143(3) is a ‘detailed assessment’, also termed as ‘scrutiny assessment’, which is conducted to confirm the correctness and genuineness of various claims, deductions, etc., made by the taxpayer in the ITR, i.e. to confirm that the taxpayer has not understated the income or has not computed excessive loss or has not underpaid the tax in any manner.
However, the assessment u/s 144 is made as per the best judgment of the Assessing Officer, based on all relevant information available with him in the cases where the taxpayer fails to comply with specified requirements, like non-filing of ITR or non-reply of IT Notice, etc.