The retrieval field is only the level of document retrieval, such as library book retrieval, which can be retrieved separately or in combination by title, author and publishing house. Search terms are similar to keywords in search engines.

For example, in library book retrieval, only the author is regarded as the only retrieval field, and the input author name is the retrieval word.

It should be noted that "Subject" is a combined search field, and selecting "Subject" means searching for qualified documents in "Title+Keyword+Abstract".