locally numbered fields

[Springer, Paul]

I have been reading through Volume 1 of Dan's book and I have a question.

 

He talks about using locally numbered fields to easily loop through the
fields on a card. He explicitly talks about not using the field's name or
ID. I don't understand how this mapping occurs; how do you assign local,
temporary IDs to fields so that you can then refer to them as "field x"
(where x is one of these local IDs) within the script?

 

-Paul