QAM and constellation diagram
Each bit can be logically coded how the constellation designer likes.

When the RF signal is physically made, there will need to be a mapping between logical position values, and physical DAC values. 

There can only be a simple 1:1 mapping between DAC and logical values for a square constellation, a 2x2 4QAM or 4x4 16QAM for instance. 

In this case, the constellation is obviously a 6x6 square, missing the highest energy corners, to truncate it down to 32QAM. This reduces both the peak and average power, for the same separation between constellation points.

Note the diagram has contours for the decision points for i3 and q2. These are obviously being set up to be 'more reliable' bits, to be used as such in the decoding, or as fallback baits when the channel is degradeed and neds to work at a lower bit rate.