UI is a bit confusing because when weather file albedo checkbox is checked, spatial albedo uses weather file albedo and UI inputs in matrix do not apply. Is this intended behavior?

It is the intended behavior. If "Use albedo from weather file if available" is checked, then the (single) albedo from the weather file is used if present, on a timestep-by-timestep basis. If an albedo value is missing in the weather file for a given timestep, then the spatial albedo is used (if "Use spatial monthly albedo values" is selected). I could see this functionality being useful for parametric studies of different years or locations that include differences in snow cover or crop harvesting (which can also occur at a sub-monthly fidelity). This will be explained in the help, but if this feature is too confusing, it can just be removed. @janinefreeman ? @cpaulgilman ?

The Lifetime Hourly output for the rear irradiance after reflection is calculated from the corresponding Matrix output, but I wouldn't think it any more confusing than both the Single and Annual energy values that SAM has for some outputs. It's also a final irradiance value after losses, so it's a metric used more than others for comparison and analysis, for example to the front total irradiance after reflection. @cpaulgilman , what would you like with this?

Can these variable names indicate that they apply to the rear surface?

Yes, done.

This is what I found so far in SAM, still working through ssc.

Thank you @mjprilliman !

The Lifetime Hourly output for the rear irradiance after reflection is calculated from the corresponding Matrix output, but I wouldn't think it any more confusing than both the Single and Annual energy values that SAM has for some outputs. It's also a final irradiance value after losses, so it's a metric used more than others for comparison and analysis, for example to the front total irradiance after reflection. @cpaulgilman , what would you like with this?

I'm fine with leaving the variables as they are just wanted to talk it through.

UI is a bit confusing because when weather file albedo checkbox is checked, spatial albedo uses weather file albedo and UI inputs in matrix do not apply. Is this intended behavior?

It is the intended behavior. If "Use albedo from weather file if available" is checked, then the (single) albedo from the weather file is used if present, on a timestep-by-timestep basis. If an albedo value is missing in the weather file for a given timestep, then the spatial albedo is used (if "Use spatial monthly albedo values" is selected). I could see this functionality being useful for parametric studies of different years or locations that include differences in snow cover or crop harvesting (which can also occur at a sub-monthly fidelity). This will be explained in the help, but if this feature is too confusing, it can just be removed. @janinefreeman ? @cpaulgilman ?

Let's keep this as is for now, and I will address this and some other albedo confusion in https://github.com/NREL/SAM/tree/pv-albedo-fix.

The Lifetime Hourly output for the rear irradiance after reflection is calculated from the corresponding Matrix output, but I wouldn't think it any more confusing than both the Single and Annual energy values that SAM has for some outputs. It's also a final irradiance value after losses, so it's a metric used more than others for comparison and analysis, for example to the front total irradiance after reflection. @cpaulgilman , what would you like with this?

I'm fine with leaving the variables as they are just wanted to talk it through.

Those "POA irradiance after..." outputs are confusing -- I think that should be addressed separately. (I think it might be clearer to show a single POA irradiance value and show the losses instead of a series of reduced POA values.)