What is data trapezoid form?

For one of our class project we are required to do the following:

Download U.S. real GDP in data trapezoid form (vintages)

I tried to search online for what does the data in trapezoid form means but could not find any answer, the trapezoid form is not mentioned in the textbook either. Does anyone happens to know what that term means?

The brackets behind the term say “vintages” and I managed to find a lot of datasets with vintages of real GDP such as this one from Eurostat but I am not sure if “vintages” are the trapezoid form because in none of the datasets that mention GDP vintages there are any mentions for the “trapezoid form”