12B is required when you switch your job in the middle of the financial the year with respect to details of income earned from the previous company
and tax deducted at source on such income if any. The reason is that
you joined in the middle of a financial year, there has to be a
correction made in terms of consolidated income earned and submission of
Form 16 from both the employers is a must.
12BA is a detailed statement which is issued by the employer and has
the complete details on perquisites received by an employee. The
employer is required to issue Form 12BA along with Form 16. Both Form
12B and 12BA come under the same master circular of Rule 26.
Form 12BA, you can download it from the official website of the Income
Tax Department of India – incometaxindia.gov.in. It is available in a
pdf format which has to be printed, duly filled with all the details and
then submitted to your current employer.