The wording in Archicad 29 Help - Merging files is missleading as it currently stands. I have gotten the following confirmation from Graphisoft:
After testing, I found the following.
If you merge two projects, the program tries to add the attributes with their own index if that index slot is unused. If it is not, then it will add it to the LOWEST FREE index slot, which could be between two occupied slots or at the end.
In the documentation, append indeed could be changed to insert, as it is a more accurate word for the situation. If you or the customer wants the description to be updated, you can submit a wish for it at the following page:
Wishlist - Graphisoft Community
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I am therefore submitting a request to change the working from append to insert as reccomended.
Wish delivered