When the researchers are doing a research, they might produce different reports from their report. For example they might register their proposal or protocol in a register, they also might publish their proposal/protocol in a journal, they might present their results in three conferences as an abstract, the slides, or a poster, and they might publish the three papers in different journals.
Although they have done one study, they have produced 8 reports of the same study.
While doing the systematic review, we put all the reports of the same study under one study name and we cite the Study rather than a single report to present the whole body of the work. This process sometimes called ‘Studification’.
This means we do not delete the other reports just because they are a protocol or a conference abstract but we keep them all under one study name for example Jackson 20171-8.