As you want I am here telling you about the duties for a candidate who recruited on Tax Assistant (TA) post in CBDT after SSC CGL exam.

Duties performed by TA CBDT

Tax Assistant in CBDT can be posted at two levels :
In assessment of tax
In non-assessment.

If posted in assessment he/she is supposed to do :
The work related to assessment of the income tax of an individual or a partnership firm or a company or as the case maybe;
He/she is to feed that data in the computer.

He/she could also be asked to do any clerical work like:
Work related to diary and dispatch, noting, drafting etc.
He/she could also be asked to accompany the RAID team.

If posted at a seat that belongs to non-assessment he is supposed to do :
only clerical work.
like to put up the file for any new Registration/Cancellation of license or any other permissions.

Promotions & Department Exams

TA to Senior TA in 3-4 years,
STA to ITI 3-5 years,

Candidate have to clear Departmental Exam for becoming ITO(Income Tax Officer).

Time taken to get promoted to ITO generally 6-8 years.

After becoming ITO, it is a long wait of around 10-12 years to become ACIT(Assistant Commissioner of Income Tax) .

Then around 4 years to DC(Deputy Commissioner) then 4 years to JC(Joint Commissioner) and around 5 years to Addl.