Yup, the "slider" can be adjusted while you play on the PU. Want to start off getting your bearings? Go full PvE and never encounter actual players. Want some extra challenge here and there? Set the slider somewhere in the middle. Want the most realistic* and difficult experience? Then go full retard PvP mode.

Basically the slider determines if/how many players show up in your areas while you're flying around. They higher you set the PvP slider, the more likely actual players will "bleed over" into your PvE universe.

* Realistic in that you are always fighting actual people, complete with the unpredictable and unique experiences this will bring.