The bobbin case is the holder for the lower thread spool in so-called central bobbin winders and circular feeders and is made of metal. The bobbin case holds the lower thread spool in position and can also be used to adjust the lower thread tension.
Source:
Eberle, Hannelore; Gonser, Elke; Hornberger, Marianne; et al. (2022), Fachwissen Bekleidung, 12. Auflage, Haan-Gruiten: Verlag Europa-Lehrmittel, S. 213