Unfortunately it is a bit of a mess with the PCA. There are two implementations of it. And they are not accessible from everywhere. Sometimes it's the one, sometimes the other.
But if your result contains the bands response, error and flags I know which one you have used
Contains a flag which marks the pixels which where covered by the ROI mask you can specify.
The membership error for a sample. The error is computed by projecting the sample into eigenspace then projecting it back into sample space
The dot product of each basis vector against the sample. Can be used as a measure for membership in the training sample set. High values correspond to a better fit.
The other PCA can be found if you use the Graph Builder.
However, we need to clean up the mess and document the PCA.
Actually it is already known since a while. We have it in our issue tracker