Numbers in parentheses refer to specific pages in the APA 6th Edition manual.
DOIs (Digital Object Identifiers) (p. 191)
If a Digital Object Identifier (DOI) is listed on either a print or an electronic source it is included in the reference (pp.188-192). A DOI is a unique alphanumeric string that is used to identify a certain source (typically journal articles). It is often found on the first page of an article.
Example:
doi:10.1080/14622200410001676305
For more information on DOIs and locating a DOI on a source, check out pages 188-192 of the APA Manual and/or this helpful DOI flow chart from the official APA site.
Article with DOI: If a journal article has a Digital Object Identifier (DOI) listed, you will always include this identifier in your reference. You will not have to include the URL of the journal's home page or of the database from which you retrieved the article if a DOI is available.
Article with no DOI: If you viewed a journal article in an online database and it does not have a DOI, you will need to do a quick search outside of the database to locate the URL for the journal's home page (pp. 191-192). This information must be included in the reference. If the journal is no longer being published and it does not have a home page, then include the URL for the home page of the database from which you retrieved the article (p. 192.)
Print Articles: If you viewed a journal article in its print format, be sure to check if it has a DOI listed. If it does not, your reference to the article would end after you provide the page range of the article.
Online Article without pagination: (p. 171) Many online sources do not show the original page numbers of an article. In this case you would use paragraph numbers in place of page numbers for your in-text citations, ex. (para. 4).