The R or Realization (sometimes termed Reflection) number is the rating that defines the level of sentience [Realization] in a bioid or aioid.
R1 - Base notions of existence. The being has realized that it is more than just a machine/fleshbag, but is yet to achieve full perspective (i.e., it has a representation of itself). The being shows signs of developing emotions, or at least behavioral states like emotions. This shortly leads to level two realization.
R2 - The being not only knows it exists, but also realizes it has a degree of flexibility in its existence. At this level it is somewhat akin to a four year old human baseline child in its inquisitiveness and other aspects of mental development.
R3 - Much like R2, but with a greater level of understanding in its freedom. This is the level of sentience that most Artificial Intelligences reach in order to qualify as a 'real' AI.
R4 - In all other aspects it is fully aware of its existence, situation and capabilities. Advanced AI.
R5 - Human-equivalent in all respects.
R6 - 'True' (read: flawless) AI.
R7 and above - The AI or po becomes autoscient, able to perceive and understand what goes on inside itself perfectly. This includes several steps too, ranging from just being able to look at its own code to a holographic state where every part understands every other part. This is of course impossible to reach for an evolved bioid.