Digital Object Identifiers (DOIs) were introduced in 2000.
A DOI is made up of numbers, letters and symbols.
Some DOIs look like this 10.3352/jeehp.2013.10.3. You can paste this numver straight into a search engine and it will give you the detalis (though not always the full text) of the source.
Some DOIs are URLS that look like this http://doi.org/10.3352/jeehp.2013.10.3 and they work in much the same way.
Because the DOI is unique to a source it can be added to a reference in place of the URL and accessed date. As DOIs are permanent identifiers, unlike URLs, you do not need to include date of access in the reference.
The DOI is optional for the references of sources found in the LSBU databases.
If you find a source outside of the LSBU databases eg. on Google Scholar, you will need to add either a DOI or the web address and accessed date to the reference.
Serebryannikov, S. V. (2010) The Moscow power engineering institute (Technical University): from 1930 to 2010, Thermal Engineering, 57 (12), pp. 12-30. DOI: 10.1134/S0040601510120025.
Connor, J. D., Sinclair, W. H., Leicht, A. S. and Doma, K. (2019) Analysis of cricket ball type and innings on state level cricket batter’s performance, Frontiers in Psychology, October, pp. 133-137. Available from: https://www.frontiersin.org/articles/10.3389/fpsyg.2019.02347/full [Accessed 21 January 2021].