I had the same question and I figured out the answer! I am looking into FAOSTAT> Value of Agricultural Production (http://www.fao.org/faostat/en/#data/QV) and there they have:
- Gross Production Value (constant 2014-2016 thousand US\$)
- Gross Production Value (constant 2014-2016 thousand I\$)
So, my understanding is that in the first one (constant... US$), the Gross Production Value is given as if the exchange rate between local currency, say EUR, and USD was pegged to the average 2014-2016 FX (... and also remove USD inflation).
As for the second one, I was quite confused...
The World Bank's data helpdesk says something that sounds just like PPP:
An international dollar would buy in the cited country a comparable amount of goods and services a U.S. dollar would buy in the United States [...]
Also Wikipedia's definition at the top sounds just like PPP to me:
[...] is a hypothetical unit of currency that has the same purchasing power parity that the U.S. dollar had in the United States at a given point in time [...]
The "Eureka" moment has come to me when looking at the calculation explanation in Wikipedia: https://en.wikipedia.org/wiki/International_dollar#Description_of_Geary-Khamis_system
It is not just a PPP, where you keep an exchange rate constant. It is also averaging all prices of the same good across countries, worldwide. And this is confirmed by the FAO here: http://www.fao.org/waicent/faostat/agricult/indices-e.htm
[...] expressed in so-called”international dollars”, are derived using a Geary-Khamis formula for the agricultural sector. This method assigns a single “price” to each commodity
So the Geary-Khamis method is used mostly (if not only) in economics to derive a single worldwide price for commodities. And because the formula also uses PPP, then you also get the same price across time.
I doubled checked with FAOSTAT data.
a) I got the "Gross Production Value (constant 2014-2016 thousand I$)" of a commodity (select all countries, many years) here: http://www.fao.org/faostat/en/#data/QV
b) I got the "Production Quantity" for that commodity (select all countries, many years) here: http://www.fao.org/faostat/en/#data/QC
c) For each country/year, I divided (a) by (b), and (as expected) I got the same number everywhere. Well, almost... for all countries with big "Production Quantity" volumes I got the same number, but for countries with smaller volumes the figure was a bit fluctuating.
Just to be sure I understood well, I repeated the same as above, but this time for the "Gross Production Value (constant 2014-2016 thousand US$)". As expected, for each country I got the same number across all years. But a different number for each country. Again... for all countries with big "Production Quantity" volumes I got the same number across all years, but for countries with smaller volumes the figure was a bit fluctuating.