is acknowledgment u/s143 (1) is assessment by income tax dept

no it is only intimation to assessee not an assessment order. If any discrepancy is noticed by computer processing in ur filed return i.e if any difference is there filed by you & as per income tax processing software then assessee have to reply within 30 days of this intimation sent otherwise Assessing officer add the different in ur income & u have to pay tax on it.

intimation u/s. 143(1) is not an assessment