I think the basic thing if one tries to read between the lines in the ReSound literature is that ReSound’s philosophy is most of the time all mics are not working together as an array but depending on the speech/noise environment around you, different mics on different sides of your head will be doing different things, the mics on the quieter side of your head closer to the speaker will be more focused in the direction of the speech and the mics on the noisier side of your head will be more in an omnidirectional mode to pick up the greater noise on that side of your head and remove that noise, as much as possible, from what comes out the receivers, with less speech input from the noisier side of your head. That would be what ReSound touts as the “All Access Directionality” mode. The other thing ReSound touts as its “secret sauce” is something like “multiband frequency processing” - low and high frequencies are presented to the user in more omnidirectional mode to preserve time differences in sound reception between the left and right sides of your head whereas midrange frequencies are more directionally focused towards the speaker and apparently the processing to focus the sound destroys timing cues. They do say that the M&RIE receiver might be considered the new omnidirectional mode. The literature does seem to says it’s mostly used in quiet situations whereas the mics behind the ear dominate in noisy situations but they don’t say why or exactly how the transition occurs. They also say that all premium HA’s use some form of “multiband frequency processing” - claim ReSound’s is superior and don’t say why. So it seems like the usual whitepaper type of stuff - lots of claims with insufficient clear explanation of the mechanics (trade secrets, no doubt) to back up exactly what’s going on. Here’s a transcript of an Audiology Online course by ReSound entitled Comprehensive Hearing in Any Environment with ReSound All Access Directionality and Ultra Focus that provides some details but not enough to make real sense of it all.
ReSound does say that it’s M&RIE receiver (MM in datasheets) is only for people with mild to moderate hearing loss and the existence of recoverable high frequency hearing is very important to recover the high frequency speech localization cues from multiband frequency processing that will give the extra umph to speech recognition in noise - if I have read and interpreted correctly what they are vaguely saying.
Edit_Update: See following post for illustration from ReSound Feature Demonstration media on how the M&RIE microphone in the ear works in conjunction with the BTE mics on ReSound One HA body in different speech-in-noise situations: GN Hearing introduces ReSound ONE - #123 by jim_lewis
Also, I think the audi says in the Audiology Online course that I reference in the link just above that their studies show that they gain on average 2 dBs advantage in speech recognition by favoring the less noisy side as opposed to a strict binaurally focused unbiased beam, when there is a significant difference in noise between one side and the other, but the idea, just as for Oticon with the OPN, is to not throw away environmental awareness and go with a highly focused beam all the time, unless absolutely necessary for speech recognition.