When to use Rigol 50 Ohm thru feed oscilloscope adaptor DS1054Z

[Ecklar]

Under what situations would I need or use the Rigol 50 Ohm thru feed adaptor on the DS1054Z ?  Is there a particular frequency point above which I would need it and below which I would not?  I saw the excellent YouTube that showed without an impedance match (low going into scope high) you might get reflections, but, I didn't understand is there a frequency threshold or some other issue. 

Thanks

[Mechatrommer]

i'm surprised this went unanswered after 6 months, maybe wrong section....i hope the OP is already know the answer, anyway without acting like a guru and repeating the fact again... fwiw some of it...
https://www.avtechpulse.com/appnote/techbrief12/
https://en.wikipedia.org/wiki/Electrical_termination