The CWTS Core Source filter in OpenAlex can be used to return only publications from sources that are included in the Centre for Science and Technology Studies (CWTS) list of Core sources. CWTS designed this list to subset the list of sources from OpenAlex included in their Leiden Ranking Open Edition. By including this filter in OpenAlex, users can further explore the data underlying those rankings or negate the filter to understand what is being excluded from those rankings.
This is not related to the Web of Science Core Collection, which is a selective list of journals indexed in Web of Science that is maintained by Clarivate.
More information on the CWTS Core sources can be found here: Core sources and core publications in OpenAlex