Götz Echtenachersaid: “I think that the 2D CPs are projected according to the active alignment”
Yes, I’d agree - and that means that when manually placed they are (or have same status as) artificial Features, which are then, as you say, “projected according to the active alignment” to define a 3D Tie point, which is then Reprojected to form 2D Tie points on the picture planes.
This means that CPs as placed are not quite same as the resultant 2D Tie points - close beside but with Reprojection error of so many pixels distant, as counted on the picture plane.
Götz Echtenachersaid: “it is absolutely possible to get CPs with 0.0 px error”. Truer to say that you can place a CP with a resolution of 1px - but I’m sure that RC’s geometry engine is working at resolution very much finer than 1px.
On recognising a Feature, which must be a pattern spread over several pixels, RC’s algorithm must decide on a (centre?) point of that pattern, to use - which needn’t be the centre of any pixel. Hopefully RC decides on exactly the ‘same’ centre point for all of the 2 or more slightly differing views of that pattern, that it can see on 2 or more photos. Unless it achieves that perfectly, that’s straight away a source of eventual REprojection error.
RC must be locating those centre points at resolution far finer than 1px, or even 0.3px, if after round-trip projections back (at 5o to 30o convergence) to a 3D Tie point and forward again to 2D Tie point, it’s achieving typical Mean and Median Reprojection error figures typically in the 0.3 to 0.5px range.