Understanding the ArXiv Identifier

I'm somewhat sad that they dropped the archive and subject fields (i.e. physics.optics). Those were really quick to grasp at a glance. And it seems they'll run out of numbers in about ten years, when monthly submissions reach six figures! (https://arxiv.org/stats/monthly_submissions)

BTW, the link has a typo - it is https://arxiv.org/help/arxiv_identifier