Conditional rule in an output style

Is there a way in a style (e.g. NIH) to tell the style to put in one field but if this field is empty to put in another field? I am  trying, in the NIH output style, to have Endnote enter the PMCID if there is one in that field but want the PMID to be added if there isn’t a  PMCID. Thanks for considering my question.

there is an option, not sure it is exactly what you need, but see the output style settings as shown in the attached image.  The optional field is editable so you can point it to the alternative field of your choice.