APA 7th Edition Style Guide

Source: the name of the "package" that the citation is published within.

some examples of sources:

• The New York Times
• Scientific American
• Yoga Journal
• a textbook titled Principles of Biochemistry 
• Proceedings of the National Academy of Sciences

Article or chapter titles are contained within a source. For example:

• The encyclopedic entry titled Anxiety Disorders is found within the source Diagnostic and Statistical Manual of Disorders
• The book chapter titled Mechanics of Style is found within the source Publication Manual of the American Psychological Association
• The newspaper article titled How Coronavirus Hijacks your Cells is published in the source The New York Times

Formatting of sources and titles will depend on what type of source you are using (e.g., a journal, a website, a book, twitter) and whether the object is an article level title or a source.

For example, if you are citing a book, then the source will be italicized:

Hatfield, H., Sivakumar, M., & Pueger, J. (Eds.), Agroclimatology: Linking agriculture to climate. American Society of Agronomy, Crop Science Society of America, Soil Science Society of America. https://doi:10.2134/agronmonogr60.2018.0006

If you cite a single chapter within this same edited book, the source is still italicized.

McDonald, M.R., and Warland, J. (2017). Vegetables and climate change. In Hatfield, H., Sivakumar, M., & Pueger, J. (Eds.), Agroclimatology: Linking agriculture to climate (pp.327-341). American Society of Agronomy, Crop Science Society of America, Soil Science Society of America. https://doi:10.2134/agronmonogr60.2018.0006

Similarly, when you cite a journal article or a newspaper article, the source is italicized while the item title is not...

Note: For a chapter in an authored book (not one with editors), create a reference for the whole book and provide the chapter number with the in-text citation only.