I think it's in "course_completion_aggr_methd"
https://github.com/moodle/moodle/blob/87afa4d7595eeafdf9e3f83d53272ee61fb66780/completion/completion_aggregation.php#L43
because this class 'completion_aggregation' is indeed called by the form
https://github.com/moodle/moodle/blob/87afa4d7595eeafdf9e3f83d53272ee61fb66780/course/completion_form.php#L67
https://github.com/moodle/moodle/blob/87afa4d7595eeafdf9e3f83d53272ee61fb66780/completion/completion_aggregation.php#L43
because this class 'completion_aggregation' is indeed called by the form
https://github.com/moodle/moodle/blob/87afa4d7595eeafdf9e3f83d53272ee61fb66780/course/completion_form.php#L67
...where this is defined:
https://github.com/moodle/moodle/blob/58ba94a7290940391d1581e591a64fc7e6bb410a/lang/en/completion.php#L184
https://github.com/moodle/moodle/blob/58ba94a7290940391d1581e591a64fc7e6bb410a/lang/en/completion.php#L184
| ... |
| $string['overallaggregation_any'] = 'Course is complete when ANY of the conditions are met'; |