In contrast to author-date systems, citation-sequence systems simply use a number for the in-text reference. This allows for many citations even in a single sentence without making the sentence difficult to read. Because of this many sciences and engineering journals prefer citation-sequence.
Traditionally citation-sequence systems used bracketed numbers, but many journals are moving in the direction of using superscript numerals instead. Again, be aware of the style used in your career field and specific publication. Here are examples of both:
Jones1 findings suggest that Godzilla has not recently terrorized any islands.
It seems that Godzilla has not recently terrorized any islands.1
Jones’  findings suggest that Godzilla has not recently terrorized any islands.
It seems that Godzilla has not recently terrorized any islands .
Notice the placement of the citation. If the author is named in the sentence the citation goes next to the name. Otherwise it goes at the end.You add numbers in numerical order unless you refer to the same source, then you use the original number. For example if you see the citation numbers 1, 2, 3, 1, 4 this means the author used source number 1 twice in the paper.
Because the in-text citations are numbered, you simply need to jump to that number in the end references to find the information for the source. Here are examples of citation-sequence end references:
1. Suzuki, I. Rampaging monsters. 2nd Ed. New York, NY: Kaiju Imprints, LTD.; 2017.
2. Jones, L. An update on Godzilla’s rampage: Who is in danger? Journal of Scientific Research on Kaiju 2016;8(3):12-36.
3. Telemann, G. Crisis in Tokyo: Godzilla returns once again. Kaiju News 2018 May 3 [accessed 2019 Jan 15]. http://www.kaijunews.com. doi:10.1136/kjn.3307500.119.
Writing tip: If the author uses source number 1 multiple times in the paper, they would need to use the number 1 each time as an in-text citation. In the end references, however, the author would only need to list the source once.
Many writers find they prefer citation-sequence systems to author-date, but the choice is ultimately up to the publisher not the author. Be sure to research your career field and be wary of stylistic requirements of publishers.