Usually information about the manufacturer of the film and/or a sequence of numbers (every foot), in the perforation area.
The footage numbers are also known as ‘edge numbers’, ‘key numbers‘, and ‘stock numbers’.
On 16mm film they are not always one foot apart, but may be at some lesser interval, though they are still colloquially called ‘footage numbers’.
Latent-image edge numbers are put on by the manufacturer, and appear during development.
Printed edge numbers are placed on the film by the laboratory, and can be coded for all materials so that any number of picture and sound rolls will have the same sequence.