Field only known in exit

Hi experts,

For a custom field ZZXXXX, data gets populated in RSA3 , when I check or uncheck the field ZZXXXX in RSA6 for "field only" option.  Code has been written for this field in customer exit.

I would like to then whats is the difference between check and uncheck of this option, since data gets populated in RSA3 in both cases.

I read many threads but I am not very clear with the solutions provided.

My assumption was, if we check the field only for particular field, extractor will go and check the customer exit to populate this field. Is this correct?

1.) If correct, then how data getting populated even when I uncheck field only for the same field.

2.) How to decide whether to check or uncheck?

Please clarify.

Thanks,

Saravanan.