This should be confirmed by means of further research, but a *possible* explanation is:

formal education: education provided by / obtained in an educational system ('traditional' primary / secondary / tertiary school system) with curricula, instruction methods, grading methods, etc.) that are (at least partially) defined and monitored externally (e.g. by school boards and regulatory authorities)

non-formal education: training and education provided 'ad hoc' -- i.e. outside the scope of a formal educational system by means of specific, targeted courses, training programs, etc. with no externally defined curricula, standards, methods, etc. but usually with internally defined curricula, standards, methods, etc.

informal education: on-the-job learning, learning by doing, etc. with no defined curricula, methods, etc.