When do you use transitions and when "normal charts"?

Hello Aviation people1
could somebody please explain, because im getting confused about what i should use and when I should use what chart and why.

STARs: when a country requires you to plan and file arrivals, use STARs. This will cover your flight path in case of lost communications.

RNAV Transitions: when you actually fly the plane and normal operations persist, expect and prepare RNAV transitions.

Thank you! That was very helpful!