In what case(s) is the end of the offering period (when one purchases the shares) considered the grant date?

I read on https://fairmark.com/compensation-stock-options/employee-stock-purchase-plans/dispositions-of-espp-stock/ in the context of employee stock purchase plans (ESPP) in the United States:

For this purpose, the grant date is normally the beginning of the offering period. Tax regulations issued in 2009 specify that in some situations the end of the offering period (when you purchase the shares) will be considered the grant date. Your company should inform you if this is the case.

In what case(s) is the end of the offering period (when one purchases the shares) considered the grant date?