STATUS_REG_AUX relates to the auxiliary ADC. For some strange reason, ST sometimes refers to ADC1, ADC2, ADC3 as 1-axis, 2-axis, and 3-axis. Probably because ST expects these ADCs to be connected to an analog accelerometer or gyro with those axes.
I originally suspected these might be different names for the same thing, but found both I1_ZYXDA and I1_321DA in CTRL_REG3, so they must be unique.
The app note has some more info about this register, but you won't find it by searching for STATUS_REG_AUX because it's annoyingly referred to as STATUS_AUX in the app note.
The conversion status is reported in the STATUS_AUX (07h) register; when auxiliary ADC data are ready, the 321DA bit of the STATUS_AUX register is set to 1.
Also, in case anyone from ST finds this post, here are some other suggested improvements for the datasheet (DocID17530 Rev 2)
- 8.24 INT1_DURATION (33h): "... duration of the Interrupt 2 event ...". Should be "Interrupt 1".
- The
ODRacronym is used all over the datasheet, but it is never defined as "Output Data Rate". Maybe this is obvious for most users, but it took me some additional digging.