Most Article types filters represent MeSH Publication Types. Publishers are permitted to include them in their submissions to PubMed, therefore your search results may not be limited to indexed records when applying these filters. However, your results may be better once a record is reviewed by an indexer and the article type(s) added or verified.
The Systematic Reviews filter is NOT a MeSH publication type. There is no systematic review publication type in MeSH. The filter runs a search strategy.
Multiple selections are added to your search with "OR," expanding your retrieval.