What does the /users endpoint look for when sorting on modified date?

For the /users endpoint in the Stack Exchange API v2.3, what does the `modified` filter in `sort` field look for?
It is unclear from the [API documentation](https://api.stackexchange.com/docs/users).

1) Does modified date include any change triggered by system or user (change in user details or change in reputation by the system)?
2) Does it includes only the user made changes like details change or something?
3) Does it includes only the System triggered changes like changes in reputation, badge count etc?
4) Does it includes any changes in user profiles?

More precisely, I am interested to know whether `modified` date includes system triggered changes (reputation change, badge count etc) or not?