1. Income tax is Einkommensteuer. A SATZ is a rate. To calculate taxes, you first come up with a taxable income, then apply to the first X dollars the LOWEST tax rate, then to the next Y dollars the rate for the next bracket up, etc., in progressive tax systems. The EingangssteuerSATZ is that lowest rate. As Mickey said, that is the rate paid on the TAXABLE income for people with enough income to have some of it be taxable, but not so much that they go into the next rates on their last dollars. Namely, their taxable income is less than X.
So your answer is NOT income tax or marginal tax rate, which is the rate you pay on your most heavily taxed, last dollar earned. It is the initial rate applied to your first taxable dollars. (If you have marks or pesos, ditto.)
2.Disposable income = verfugbares Einkommen is the money where you have some real say in what you do with it month after month. After you've paid rent or mortgage or whatever and all you need to scrape by. It's what you use to finance what you want but don't necessarily need. Best Wishes. Nancy