The 8-page spec sheet doesn't go into much detail. It does call the tuning input pin an "Ageing Adjustment" and that implies that it's something that is tweaked very slowly over the product's life through some high level process.
If you're dealing with a large production run, then they offer a version where the Ageing Adjustment is not included. You would decide if you need the feature or not.
If you're dealing with a little one-off project, then design the PCB so that you can experiment with various options. The Ageing Adjustment is DC so you can easily play around with jumpers and variable resistors. You would probably want to make sure the DC signal you apply (no matter if it is hard-wired to ground or to a certain fixed voltage) is clean and stable.
The 8-page spec sheet didn't give much more info about that input. Maybe I missed it, but I didn't even see the input limits.