The world is 3D, but everyone forgets the vertical coordinate reference system (yes, the one for the Z coordinate)
Digi3D.NET is purely 3D, and since the moment it started requesting the user Coordinate Reference Systems, it has always requested systems 3D. You have everything very well explained in the Coordinate Reference Systems page of the online help.
In addition, Digi3D.NET natively loads projects created by other programs such as Inpho or PhotoScan. Personally, I love both formats (perhaps more the one of PhotoScan), among other things because they indicate the coordinate reference system of the orientations they contain (point number 1) and also, they do it in a standard way, using Well Known Text strings, which is how it should be done (point number two), but both suffer from the same:
Forget the Z coordinate
I don't know what the poor Z coordinate has done to them, but they always forget about it...
First it happened with the sensor Digi3D.RPC, which is responsible for working with satellite sensors. A company asked us for the possibility of loading aerial triangulations made with Inpho in *UTM (*the truth is that I did not imagine that you could make RPC orientations in UTM, but yes, I smell for the developers of Inpho, because that works and well), but in the project files, the vertical component did not exist. We solved it by requesting the user the vertical SRC in case when loading satellite projects of Inpho did not appear in the WKT string the vertical component.
Actually in this case it was not so important, because the formulas of RPC do not care about the coordinates Z; no matter if they are in ellipsoidal or orthometric, there is no requirement, however we decided to request that to the user if he made any transformation to another system, to know where things are really.
In the case of PhotoScan it is also worse, because to be able to work with that format, you have to transform yes or yes to geocentric coordinates, and to transform to geocentric coordinates you must always use ellipsoidal coordinates Z, so that if the coordinates of the orientations are orthometric, I say that you will have to know the vertical coordinate reference system in which they are, right?
They could perfectly be in EGM2008, or in EGM96 or REDNAP08 or Average sea level in Alicante, or whatever. Also, maybe the coordinates Z are measured in miles or feet or microns, or millimeters... Who knows?
EDITION ON DAY 27/2/2019: PhotoScan does work with vertical reference systems as explained in https://blog.digi21.net/2019/02/26/modificacion-en-el-modulo-de-carga-de-archivos-de-photoscan/
Digi3D.NET never makes any assumption, it has no black box that makes a decision for the user, so we have added also in the sensor Digi3D.ConicSensor, which is responsible for conical cameras the same solution as in the case of the satellite sensor: Now the program requests the vertical SRC in case the aerial triangulation file you are loading provides one but it is in 2D.
In the video below you can see it in action: