Distance has an important impact on many aspects of game-play.
Diplomatic actions are restricted by distance, and is also an important modifier in several diplomatic actions. In war, it is important as it impacts the speed of armies and fleets, and it impacts many other aspects of game-play as well.
The distance between two land provinces can be viewed in tooltips, by clicking on province and then hovering another.
Diplomatic actions are limited by distance. Most require being within diplomatic range: 700 distance from their nearest controlled province. Having the same religion, culture group and culture as the target of your action extends the diplomatic range by 100 each.
Some diplomatic actions have stricter requirements:
AI behavior is also affected by distance:
Province distance affects the time it takes armies and fleets to move between provinces (see Warfare#Movement_speed).
The mercenaries a ruler can hire is determined by the distance between the ruler's capital and the mercenary's capital.
The price of a merchant republic trade post is increased by 30% times the distance from the closest controlled holding. See Trade_post#Merchant_republic_trade_posts for details.
The titles that adventurers target are also dependent on distance. The casus belli that non-claimant adventurers use is required to target a title within a distance of 800.