Section 40A of the Income Tax Act specifies certain expenses that are not allowed as deductions when computing taxable income. This includes excessive or unreasonable payments made to relatives, and cash payments exceeding Rs. 10,000 for most expenses or Rs. 35,000 for vehicle leasing expenses. There are some exceptions such as payments made to banks or the government. The section aims to prevent overstating expenses to reduce tax liability by disallowing fabricated payments or excessive related party transactions not at arm's length.