I think it helps to get concrete so I'll try to enumerate various types of id that arXiv uses. I use
id for our local ids:
- id and arXiv:id - our local internal ids. These are used internally and also quite widely in published literature. They are well recognized by the community. They are not resolvable and not guaranteed to be globally unique. For several years we have been encouraging people to add "arXiv:" as a prefix when printed. This increased the accuracy of pattern matching and is especially important now we have dropped the subject area prefix (i.e. we use arXiv:0808.0001 and not the old arXiv:cs/0610031). There is some discussion at http://arxiv.org/help/arxiv_identifier including the versioning scheme.
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