Core

A cylinder, of various plastics, wood, or metal, on which a roll of film is wound. Film Cores usually come in three sizes and various gauges:

• 2” rarely used for archival purposes, mostly used commercially for 35mm storage or projection purposes. (various colours)
• 3” used in archives and commercially. Archival preference for this size because it reduces damage due to curl .
• 4” used for storage of polyester based magnetic sound tracks.

https://www.nfsa.gov.au/sites/default/files/uploads/2010/10/06/cores.jpg