Do you mean "Delineate fields on current record only"?
That option tells FileMaker only to show field backgrounds on the current record. It's a way of giving an additional indicator of which record has the focus, especially in List View:
If that's not what you mean, then I'm missing something; just let me know what it is.
Turning this option on seems to control the fills applied to specific fields on the layout. I tested this in list view and found that the fill only showed on the selected/current record in list view, and changing records showed the field fills only in the newly-selected record.
I did not get it to control lines applied to the fields by record, which seemed more in keeping with its name.
The only info I found was in the FM Help system at:
Home > Designing and creating databases > Creating and managing layouts and reports > Editing layouts > Showing field borders and fill for the current record
It seems to be a LIST view option only. I think that why it is confusing. Now is there a way to tell what record is the current record?
As shown in Mike's graphic, in list view there is a dark vertical bar along the left side of the list.
And, of course, if you have the delineate option on, it will be the one with field backgrounds showing.
Yes, I meant "Delineate fields on current record".