I've found this on another forum:
"ATA (Archery Trade Assoc.)- Is strict. Exactly 70lbs. an exact 30 inch draw and exact 350 grain arrow. ATA is also shot with fletching or vanes on the arrow and with a D-loop on the string.
IBO- 30in +/- .75(usually +) and shot anywhere from 70-82 lbs with a an arrow that is 5 grains per pound. IBO shoots off the string without a d-loop which is a touch faster and they shoot without fletching or vanes on the arrow which cuts down on drag on the arrow so it is faster."
Also got this on another website:
"For those with questions regarding ATA/IBO velocity: ATA speeds are 70 lbs, 30 in. draw at 5 grains per lb. IBO speed is commonly believed to also be tested at 70/30. However, in actuality, the IBO standards allow for speeds to be tested at poundages as high as 82 lbs as long as the arrow is at 5 grains per lb."