Does it matter, whether eClass 4 is mapped to eClass5 or the other way around?

Yes, it does matter. This shall be explained with a small example:

eClass 4 class 01-02-03-99 has been splitted in:

Then a mapping from eClass 5 to eClass 4 could work automatically (all projects, which use one of the four eClass 5 classes are classified to the one eClass 4 class).

But a mapping from eClass 4 to eClass 5 would not work, because the mapper does not know, which of the four classes should be taken.

In order to solve theses cases, the mappings from eClass 5 -> eClass 4 and eClass 4 -> eClass 5 are two different mappings.