Most digital servo's are limited to 333Hz due its possible max pulse width of about 2.1~2,2 msec (150% EPA setting) and you need to have a certain time low pulse tot tell the servo to reset and wait for the new pulse. Sanwa did stretch this (I believe SHR called) mode up to 384Hz because there is some room to shorten the low pulse.

But then the higher speed servo's in SSR mode use a center pulse width of 300 usec with the same 384Hz frame frequency, with this they could lower the latency by a little bit more than 1 msec. About SXR and SUR I do not know much than only SXR still uses the same 300 usec center pulse but with a higher framerate.

Futaba its SR works on an 833Hz frequency and have a center pulse of 760 usec, the SR mode of the FlySky transmitter goes up to 1000Hz because as with the Sanwa slight higher frequency there is the room for it. Futaba UR is at 1600Hz with a center pulse of 190 usec

Providing any signals a servo is not made for will result in a weird behavior, like putting a 1000Hz on a 333Hz limited servo is basically is impossible with a needed pulse width of 1 to 2 msec. What is possible and I think done with some transmitters is only sending the info repeated on a much higher rate but the receiver puts out the signal limited to the servo, In that case the receiver can determine what the right info is among some disturbed data.