The SA103 is a form used as a supplementary page to the main SA100 form when filing a self-assessment tax return in the United Kingdom. The SA103 form is used to report specific types of income such as:

1. Interest and dividends earned from investment and savings accounts
2. Income from trusts, estates, and settlements
3. Income from UK property and furnished holiday lettings
4. Income from patents, trademarks and copyrights

The SA103 form requests details of the income, such as the name of the institution or company where the income is coming from, the rate of interest or dividends paid, the gross income before and after the deduction of the allowable expenses, the amount of tax paid by the institution, etc.

It's important to note that if an individual has more than one type of income, they will need to complete multiple SA103 forms to report all of their income. If you have questions about the SA103 form or need help with your self-assessment tax return, you may want to consult an accountant or tax professional.