A customs declaration serves to express the intention to put goods through a certain customs procedure. According to the Customs Code (CC) of the European Union (Art. 61 CC) and the Implementing Provisions of the Community Customs Code (IPCCC), customs declarations can be submitted in oral, written or electronic form, or in another form of statement of intent – (for example, by using the green exit at airports). The relevant legislation indicates which form should be used in which situation.
Source: Zoll online, edited (only available in German)