On the Details tab there are two fields that are confusing for me: “Printing” (“Druk”) and “Number Line” (“Drukgetal”). If I want to register that I have the 10th edition (printing) of a book, do I enter 10 in the “Printing” or in the “Number Line” field? Can you explain?
“Number line” is something which is physically printed in a book, like “2 3 4 5 6 73 72 71 70”. It may be there or not, depending on a country.
“Printing” is a batch inside an edition. Like you have “2nd edition”, which is physically produced in 3 printings, first 40.000 copies, then 50.000 copies and then 20.000 copies. This is a technical detail, also not always provided. Note that it sits next to “Printed by” field - each printing can be done by a different printing house.
Thanks for your reply! However, this makes it even more confusing to me 
I use the Edition field to indicate whether a book is a Standard Edition, Revised Edition, Deluxe Edition and so on. The Printing field is used by me to indicate the printing number within that edition (1, 2, 3 and so on). Usually each printing has its own barcode. The field Number Line is not used by me. I have no idea what it means, even after your explanation. It would have been nice if Collectorz.com would introduce Help-popups for each field, or publish a manual for each of their software.
Number line is described completely here: Printer's key - Wikipedia
Printing (also called reprint, definition confirmed here: PRINTING Definition & Meaning - Merriam-Webster) should not have a new ISBN, unless there are changes beyond correcting typographical errors (as per ISO 9707:2008 point 2.35).
I use Edition for both number and additional information: “2nd (abridged)”.
Remember you can rename the fields to give them the meaning you want. I think there is nothing wrong with your use of all these fields.
OK. Now I get it. The attribute “Number Line” does not exist in my country, at least it’s not listed in the book itself. So I will leave that blank.
And you’re right: Any user of the Collectorz software can use the fields the way they want to, at least concerning the contents of a field (except for some predefined lists that can’t be altered, of course). That’s in my opinion a big plus of this software!