What is a ‘normalized’ identifier?

We’ve introduced a new system-generated field, which expresses external identifiers (DOIs, PMCID, PMID, ArXiv, Bibcode, ISSNs, and ISBNs) in a normalized format for the purposes of matching and grouping. Normalization is done based on the rules of the identifier type, and may include setting all alpha characters to lowercase, or transforming spaces, dashes, periods and other characters that can be treated as equivalent. It also adds standard prefixes and suffixes as appropriate. For example, https://doi.org/10.1/123, 10.1/123, and https://dx.doi.org/10.1/123 will all appear in this field as https://doi.org/10.1/123.  The existing identifier value is unmodified.