Understanding the relation between A/F Ratio, A/F Base and O2 signal

[SailorBob]

I'm used to working with OBDII, and it's not completely clear to me how A/F Ratio, A/F Base and O2 signal relate to and influence each other.

The car in question is a 99 Nissan Almera GA16DE. Looking at my log files, it seems like A/F Ratio is the ecu's interpretation of the O2 signal. In obdii land, whenever the O2 signal crosses 0.45v that's considered a switch between lean and rich or visa versa. However, from what I'm seeing in the log, it looks like AFR only switches when the signal either goes above say .8v or below .2v or something like that.

Regarding AFB, is it also an interpretation of the systems current state, or is it a target the ecu is trying to achieve? For example, it I see AFB of 98 which seems to be lean, is the ecu adding or taking away fuel?

Thanks,
Robert

[SailorBob]

Well, I figured one thing out. By adjusting the Base in active tests and watching the O2 signal I could see that numbers less than 100 are taking away fuel ( more lean ), and numbers above 100 are adding fuel ( more rich ).