How to use if conditions in geant4, restricting it to a certain volume of the world?

Let’s suppose i want that some information, maybe electric field, maybe magnetic field or other field, be restricted only to a certain region of the world volume. How could i achieve this?

I have read a code that is like this:

if (20.003*mm >= sqrt(pow(Point[0]*mm,2)+pow(Point[1]*mm,2))) {    
        .... //where ... here would contain the information to be restricted only at the positions above.

So, is that the right way to restrict a information under some region of the space? In this case, a disk. What does it means Point[0] or Point[1]?
Could it be something like that?

if (20.003*mm >= sqrt(pow(x,2)+pow(y,2))) {    
        .... //where ... here would contain the information to be restricted only at the positions above.

Generally I do that in post-processing/analysis of the file generated by G4. In your case, may be you can implement that in and use step->GetPreStepPoint() or step->GetPostStepPoint() and restrict the fetched point (x,y,z) to the physical domain of your interest?

See following URLs:

G4bool Mensdec::ProcessHits(G4Step *aStep, G4TouchableHistory *ROhist)
G4Track *track = aStep->GetTrack();
G4StepPoint *preStepPoint = aStep->GetPreStepPoint();
G4ThreeVector particle = preStepPoint->GetPosition();
float x = particle[0]*mm;
float y = particle[1]*mm;
float z = particle[2]*mm;

void Campo::GetFieldValue(const G4double Point[4], G4double field[6]) const{

    if (20.003*mm >= sqrt(pow(x,2)+pow(y,2))) {    
    else if (sqrt(pow(x,2)+pow(y,2))>20.003*mm and 91.003*mm>=sqrt(pow(x,2)+pow(y,2))) {
        auto V = -2530*volt;
        auto a = (1800*volt+V)/(log(20.003/91.003));
    else if(sqrt(pow(x,2)+pow(y,2))>91.003*mm){
        auto V = -2530*volt;
        auto b = -(2550*volt+V)/(log(91.003/96.003));

Thank you.
I have writed this. Do you think is it right? The compiler run and the visualization open as well.