One of the hidden gem of the new 3dsMax 2019 feature is the Custom Scene File Data Streams. This allows uses attach custom strings to your max and access from outside of 3dsmax. It is not only eliminate the need of copanionexternal file for such data but also opens up accesingof ,msx file realted data to any program or language.
For more details, check the help.
CustomFileStream Interface
CustomSceneStreamManager Interface
To show the power of this feature, I made a sample imementation of this feature. csMergeBy script. It allows users to merge object from other .max file by Layer, SelectionSet, ObjectID and Material name.