Pacejka's Magic Formula:Nominal vertical load on tire

The meaning of $F_{Z0}$ is the vertical load on the tire. Since tire-ground interaction is friction based, greater load leads to greater friction forces (longitudinal and transverse).

So, for example if a truck is loaded then the force on each tire is greater (compared to an unloaded one), and so are the transverse and longitudinal forces on the tire.

Regarding, whether that is static or dynamic load: the $F_{z0}$ represent a dynamic load in the sense, that when taking a turn, the wheels on the outer side of the turn are loaded more than the wheels on the inside of the turn. So most of the time, when a vehicle is turnign there is continuously redistribution of the forces on the wheels.