Zone bonus stat applies a specific modifier X when the player occupies a specific type of game zone(usually aligned to specific element such as fire bonus for lava, water bonus for sea, earth bonus for sand,air bonus for elevations,etc).
Typically at middle/center/bulk of zone type(sea,sand,grass,mud,mountain,etc) the modifier has 100% of bonus applied, with less N% when zone border is approached and 0% if the player is outside of the zone.