Open loop is when the ECU stops trying to adjust fueling to achieve a certain AFR (usually 14.7:1)... this is around 70% throttle or 2.70v. Trying to tune for any other AFR prior to that point will at best have no effect and at worst cause AFM related error codes and driveability issues. Generally I don't think you'll have driveability issues unless you're using another system which actively alters the fueling to achieve a set AFR such as the MAFT Pro, but it's best to just avoid the headaches. If you want a little overlap go easy on the enrichment you give it over stock and set the throttle level a good higher (try low 60's or above).
There are a few other cases where the ECU operates in open loop (such as high engine loads at lower throttle; like when you start boosting hard at half throttle going up a hill), but as the system operates most of the time in closed loop you should tune with that in mind with the low and mid loads and their set points.
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