Chronological order means listing writing your career summary from the very first step to the most recent one and a functional one means writing the relevant experiences and skills first then writing a brief career summary later on.
When you have to choose between the two, it will depend on your current career situation.
If you have worked in the same industry for a long time and have had a pretty regular work period, chronological resumes will work for you.
On the other hand, if you are switching directions in your career, have had a gap in between job, looking to restart your career or don’t have the normal background for the job, then you might want to use the functional format as it lists all your relevant information at the very beginning thus, helping you avoid the rejection pile.