Selecting "Subject" means searching within the scope of "Title, Keyword, Abstract", that is, all documents containing search terms in "Title, Keyword, Abstract" are detected.
Take your "endogenous" and "Stipa" as examples. In the subject, that is, in the title, keywords and abstract, there are nine documents such as "endogenous" and "Stipa". You can search in the title, keywords and abstract respectively. The number of documents detected may be less than that retrieved in the topic, which is 8. Among them, there is a document: The Effect of Endophytic Fungi on the Seed Germination of Its Associated Plants. There are endophytic fungi in the title, but there is no Stipa. There are two words in the keyword and only "needle" in the abstract.
The selection of "references" is not only in the title of the article, but also in the theme, which needs textual research.