Example of Proper Usage:

'The New England Journal of Medicine' → 'ISO-4 Abbreviation' → 'N. Engl. J. Med.' is correct, citation: https://cassi.cas.org/publication.jsp?P=eCQtRPJo9AQyz133K_ll3zLPXfcr-WXfRYSDpmM6Un_wybpAGvRMBlIRhNERAEVqAzW-Fvpktd8yz133K_ll3zLPXfcr-WXfcORL6bwBa70yz133K_ll3zLPXfcr-WXfAEuwcz0rv-z5qbpPW9O0zA.

Example of Improper Usage:

'The New England Journal of Medicine' → 'ISO-4 Abbreviation' → 'NEJM' is incorrect, as it is an informal abbreviation or acronym for the journal. ISO-4 abbreviated titles will generally contain shortened words with periods.