DIN766/A and EN818-3 (what you're calling ISO818) have 3 major differances. One is the physical size of each link, on a 10mm this is 2mm per link length. The other is the EN818-3 is 25% stronger than the DIN766/A (generally). The 3rd is the DIN766/A has far higher calibration than the EN.
Din766/A 10mm has a tolerance of +2.5/-0.5mm per 11 links where the EN has +/-1mm per link.
But saying that some EU manufacturers are uprating their DIN766/A chains to the same loads at the EN. The Maggi Group is one and I hear Marit and Devane (France) are close to doing the same. The EN818-3 is a Grade M or what was called Grade 40 (and still is in the US).
Acco HT or G4 or G40 or HTG4 are all the same chain. It is a Grade M just like the EN818-3. The biggest difference is again the physical measurements of each link. The US are still imperial.
A 10mm DIN/766 and a 10mm EN and a 3/8" US G4 are all different in physical sizing. So pick the one that fits your winch
or carnage maybe the outcome of fitting a new chain.
The US sizing is peculiar to the US only as a rule
and not that easy to fit outside of it often. That maybe a consideration if venturing far a field.
A US made G40 and a EU made G40 will break at the same time even though the US WLL are a lot higher. The US runs smaller safety
margins than the more commonly used 4 to 1. Break of a 10mm G40 is 6400kg, there abouts. On a DIN it is 5000kg.
They use made H-T, which stands for 'High Test' not high tensile, means it has had an elevated WLL, see comment directly above. It's basically a cunning marketing
A US or EU or Aussie or Sth African made G40 will break far beyond an Asian made G40. That's only because most Asian made chain is crap and not trustable. If you don't agree with that just stand in my shoes for a few weeks (sometimes only 1 day will do it) and you will change your mind.
'ISO' is not a standard, it is a bunch of people who set and maintain the standards. Many winch
companies are using the term ISO these days quite incorrectly.
So if you are in the US and planning on staying close go with the Acco/pearson G40. If in the EU and planning on going further you are better off going with a DIN or EN as it is far easier to replace around the world if you have too. Also the EU made is generally a lot cheaper.
If any where in the world and planning on using the chain for anchoring
make sure you are buying
what you think you are, ask for a Test Certificate. If it doesn't clearly state the country of manufacture in a way you believe don't buy it. If it doesn't have a Test Cert don't buy it. ALL good manufacturers have believable Certs, the shop might not have a copy but should be able to get one faxed over reasonably quickly.
DIN766/A is not the same as DIN766/B or /C just as EN818-3 is not the same as EN818-1 or -2. Physical sizing is just one difference.
All of the above applies to well made quality chains. We don't regard asian made as that. It is getting better (well some is) but generally it is still crap, not reliable, a lot weaker and a lot softer (it will stretch easily) than the good stuff.
Don't buy on price
alone, that is no indication of what the chain is like.
Buy what fits your winch.