When we have different WikibaseRepo installations on several wikis they all have their own items and item ids. These should be globally unique and identify the wiki they belong to because they might also be used within each other wiki.
Example: commons:File:xxx.jpg uses wikidata:Q123 as author
There are several options how this can be implemented:
using different prefixes (Q => Wikidata, C => Commons, Z => Wikiquote)
+ item ids do not get more complicated
- you need the mapping of prefixes to wikis on every place you handle item ids
- the mapping only works for wikimedia projects but third party users might have same prefixes for different things
adding the domain/interwiki prefix to the item ids (wikidata:Q123 or d:Q123)
+ creates globally unique identifiers in the wikimedia projects
- hard to read and not "handy", also takes more space/characters
using full urls for item ids (www.wikidata.org/entity/Q123)
+ creates globally unique identifiers throughout the web
- hard to read and not "handy", takes even more space/characters