Assessment

Not necessarily. It is triggered only when departments gets some information about concealment of income as per their data and the return filled by assessee.

Actually regular assessment is referred to assessment u/s. 143(1).

Only when it is selected for detailed assessment to verify the detailed of the ITR, notice u/s. 143(2)  is sent and the scrutiny is termed as assessment u/s. 143(3) (or even in other cases u/s. 147).