If you're referring to the line 15 on the 1040, Other taxes, it comes from Schedule 2, line 10 of the 1040. It could be any of the following: Self-employment tax Unreported social security and Medicare tax from Forms 4137 or 8919 Additional tax on IRAs, etc. Household employment tax Repayment of the first-time Homebuyer CreditIf you're not referring to line 15 of the 1040, please respond in this thread with which line in the 1040 you have a question about,.