If income cannot be attributed to any other type of income, it may fall under "income from other services". The law provides examples of other services, such as occasional brokerage and the rental of movable items (§ 22 No. 3 EStG).
Income from "other services" is tax-free if it is less than 256 Euro per year. This is not an allowance, but a tax exemption limit. This means: A total profit of 255 Euro is tax-free, whereas a profit of 256 Euro or more is fully taxable.