Initial section

Is run only once, at the start of the model run, defines

initial values of variables (at t=0) while these values can change in the dynamic section, here: MapB, or
constant values of variables, valid for all timesteps (which do not change through time), here: MapC

Is a sequence of pcrcalc statements, executed from top to bottom

Values of variables at the end of the dynamic section can be used in the dynamic section

binding
 MapA=a.map;    # input map, completely filled with value 10
 MapC=c.map;    # output map

areamap
 mask.map;

timer
 1 10 1;

initial
 MapB=5*MapA;             # MapB becomes 50
 report MapC=MapB/10;     # MapC becomes 5

dynamic
 MapB=MapB+MapC;
 MapD=MapB+1;