This is called the Citation-Name (C-N system) or the Alpha-Numeric system. It is one of the three referencing systems recommended by the Council of Science Editors (CSE, 2018). In this system, the end references are listed alphabetically by the surnames of authors and numbered consecutively. The numbers assigned to the end references would be used for in-text citations irrespective of the sequence in which they appear in the text.
Reference
CSE [Council of Science Editors]. 2018. Scientific Style and Format: The CSE Manual for Authors, Editors, and Publishers (8th Ed.): The University of Chicago Press, 840p.