A tax demand notice will be delivered to the taxpayer by the income tax department if there is a disparity between the income tax amount calculated and paid by the taxpayer while submitting an income tax return (ITR) and that computed by the department. After the tax department has processed the ITR, the taxpayer will receive the tax demand notice.

In accordance with section 143(1) of the Income-tax Act of 1961, a notification will be sent via email and SMS on the same. The tax demand notice can be found in your account on the government's electronic filing website.

"The time frame for responding to this is 30 days, in accordance with the most recent income tax laws. If you don't react to the notification within the allotted time frame, the department will move through with the recommended adjustment to your total income as stated in the notice given in accordance with Section 143(1) of the Income-tax Act. If a tax demand is made in accordance with the notification provided under Section 143(1), the notification is treated as a Notice of Demand under Section 156. Therefore, you must pay the total sum within the time frame specified in the notice."

The assessee is responsible for the following penalties if the person to whom the section 156 tax notice for demand has been issued fails to pay the amount required within the deadline:
Interest under Section 220: After the 30 days allotted under Section 156 of the tax notice have passed, interest at the rate of 1% per month, or a portion thereof, is due.
Penalty under Section 221: The assessing officer may impose a penalty under Section 221 on the assessee.

Read more at: How to Response Income Tax Notice