iCite is currently built on top of data from PubMed. For information about PubMed coverage and preprint inclusion see the PubMed User Guide.
Because searches are routed through the National Library of Medicine's Entrez sevice, they should closely, but not exactly, match the same query results in PubMed. (Exceptions include very new articles that haven't been processed in iCite yet).
Note that in some cases, PMIDs cannot be matched to citation records. In this case, a warning appears:
Clicking "View details" displays the list of missing articles and common reasons why the PMIDs were not matched:
Most often, this occurs because articles are outside the year range for available citation data. iCite currently has citation data for papers from 1995-present, but note that papers published in the most recent year (some of which are only a few months old) have not had enough time to accrue a meaningful citation count. For this reason, only articles that are at least a year old are available for calculating RCR. (Sometimes, however, newer articles do have an RCR. See here for more information.)
Recent papers may also show stronger seasonality patterns and discrepancies because of online publication preceding print publication. Papers that are published early in the calendar year or with long online-only publication periods will have a longer period in which to accrue citations.