"Fire for effect" essentially means no (more) marker rounds
needed to adjust fire. The shells are landing on top of the targets
& it's time to bomb their brains out!
FYI There's different types of arty fire missions: registration
fire, adjustment fire, fire for effect ("FFE"), harassment &
interdiction ("H&I"), time on target ("TOT"), illumination
("illum"), et al.
The arty officers who follow the infantry are either Forward
Observers ("FO", Lieutenant rank) or Fire Support Officers ("FSO",
Captain rank), assigned to COY & BN levels respectively. Unlike
the Americans, our local versions have Technical Assistants ("TA")
who carry the signal sets & may help plan/call the fire
missions over the comms. I was both a FSO & FO TA during my
NSF.
A new bird is usually trained to adjust-fire - or with prior luxury
time, registration-fire - before executing a FFE, but I suppose
immediate FFEs will be the norm come wartime.
"Fire at will"? That's for the M16's during range shooting, isn't
it?