The question of what it is in the picture.
"Variable Frequency" 
We are getting deep into the advanced SDC operation...
... so here we are, with lots of ground clutter.
We are in 75Km mode, and some clutters are from over this range.
If we switch on our lovely SDC, we find out, that it could filter clutter till 75km, but...
... all clutter, that is beyond 75km is still messing up our scopes.
If we now turn off the variable frequency, than we can filter all clutter, from the screen (over 75km also).
