Term used in the German Income Tax Act (§ 1 (4) EStG). Natural persons who are not residents of Germany are subject to limited income tax liability if they earn certain types of income (defined in § 49 EStG) in Germany. This applies, for example, to income from guest performances. Where double taxation agreements are in place between the countries concerned, these take effect to prevent double taxation of such income.
Source: EStG, edited (only available in German)