Tax Assistant in CBDT as the name suggests is an assistant level post.

You will be posted in one of the two levels; either in assessment of tax or in non-assessment.

Your work profile will depend upon where you are posted.
If you are posted in assessment, you are supposed to do the work related to assessment of the income tax of an individual or a partnership firm or even a company. In addition to that, you will be responsible for feeding that data in the computer. He could also be asked to do any clerical work like work related to DIARY AND DISPATCH, noting, drafting etc. He could also be asked to accompany the RAID team.
If you are posted in non assessment, you are supposed to do only clerical work.
This cycle of assigning of working seats keeps changing periodically.

Tax Assistant in CBEC can have three types of postings namely
1. Excise
2. Service Tax
3. Customs

Tax Assistant in CBDT career path:

SSC CGL - Tax Assistant in CBDT Job Profile, Career Path, Salary, Qualification and Stage of exam.

As Tax Assistant in CBDT, you will have the following career path

Sr Tax Assistant
Income Tax Inspector (ITI)
ITO (Group-B gazetted)
Assistant commissioner.

Tax Assistant in CBDT salary and allowances

Pay Scale
5200-20,200

Grade Pay
2400

Initial Pay
5200

Total Pay (i) + (ii )
7600

As a Tax Assistant in CBDT you will be entitled to the following major allowances:

DA on Total Pay above (around 107 %)
Transport Allowance
House Rent Allowance(if not provided with quarters)
Petrol allowance
A MTNL/BSNL sim from the department on which all STD calls on every type of BSNL/MTNL phone are free; and 400 minutes on other phones whether STD or LOCAL are free for every month. And also 1 GB 3G data free use of data on internet.

According to current pay scales, You would get a salary of around 23,000/- pm initially( can be varied a little as per place of posting)