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Purpose
Once a listing has been activated, you can no longer change it. If you want to change a listing during its validity period, you have to split it into two separate listings; one with a validity period wholly in the past, and the other with a validity period wholly in the future. This ensures that your data remains consistent so that it can be analyzed later.
Use
You can split an entire listing, or an individual item within a listing (see
Splitting a Listing Header or Item).