bool-file allows access to a specific file with boolean semantics.
Auto-connect: no
ⓘ This is a snap interface. See Interface management and Supported interfaces for further details on how interfaces are used.
The above feels a bit sparse as documentation …
Here is an example for a slot definition with this interface granting access to an led (it also allows access to gpio sysfs nodes):
slots:
[...]
led-green:
interface: bool-file
path: /sys/class/leds/green/brightness