Hi Alex, 

The only difference is the expiration time  2pm. We have decided to convert our 2pm expiration options from American style to European style for several reason. There was very little use of the early exercise process (<.2%), so when it occurred it was a pain point operationally for clearing firms and participants to make sure they early-exercised the few options they would get allocated. It also aligns us better with the larger OTC option market that is strictly European style (and we are trying to win over some of these participants). Finally, we are launching Volatility-Quoted Options in October and this will involve making calculations in our match engine with an option pricing model. As you may know, the American style model are quite a bit slower (more intensive) given their iterative estimation process, compared to a simple closed form model for European style options. Electronic trading is all about scaling and speed!