From the GMA Techline
Subject: AOS or EOS? What is the difference between AOS (Apparent Opening Size) and EOS (Equivalent Opening Size)?
Reply: The real answer is: “Not much.”
Now let me answer your question. The U.S. Army Corps of Engineers developed AOS in 1972 on the basis of sieving glass beads through woven monofilament fabrics (which was all that they used).
Then came the nonwovens, which complicated things greatly because there are not any straight-through pathways. Using the same dry glass bead sieving technique, however, the industry had to create a new term—the EOS.
Today, we use EOS for all geotextiles, nonwovens and wovens, for simplicity.