Sent from my SM-N9005 using XDA Free mobile app
The flags don't make your phone magically faster or make the cell tower have priority to you, or what other theories people have about it. The flags are for what the phone does with the APNs. If you have multiple APNs and one has the default flag, then the phone will use that APN. Same thing for the MMS flag, the phone will use that APN for sending MMSs and so on. I don't know what SUPL is, but HIPRI just makes the APN have higher priority over other ones. Hope that clears up anything, and any performance increases might just be placebo or because you are reconnecting to the tower when changing APNs. If AT&T really had an option to increase data speeds, wouldn't you think they would have that turned on by default to prove they have a faster network?