O and D scouting
So my question relates to the slider bar and not the actual play.
When scouting the the most frequently called Opponent Offense Plays does the slider bar represent how well my team defense knows that play or how well the Opponent knows their own play?

I've always interpreted it as, how well my Defense knows that Offensive play, such that if it was quite red I might not need to key on it as much if my Def is already familiar with the play and might select a different play for my Def to practise if there was one the Opponent called frequently and had good gains. But lately I've questioned my thinking

does my question make sense?

The way I think it works is that on the scout offense/defense, it is how familiar your own team is with that play. So yes, if the bar is full, your team knows that play well (and remember - it's the reverse - scout offense, means that your defense knows that play well, scout defense means your offense knows that play well.) Also, if you select a play to practice against for the scout team, and that play actually gets used against you in the game, you are supposed to get some "bonus" for practicing against it during the week. How much an effect that actually is we don't know.

OK thanks, that's kinda how I was thinking. something small but could potentially mess you up if you were thinking reverse