Explanation/Reference:
The default content access account is the account used to crawl content. This account can be overridden by specifying a differentaccount by using a crawl rule.
Learn how to specify a content access account, create crawl rules to include or exclude directories, and prioritize the order of crawl rules.
You can add a crawl rule to include or exclude specific paths when you crawl content. When you include a path, you can optionally provide alternative account credentials to crawl it. In addition to creating or editing crawl rules, you can test, delete, or reorder existing crawl rules.
Before end-users can use search functionality in SharePoint 2013, you must crawl or federate the content that you want to make available for users to search.
Effective search depends on well- planned content sources, connectors, file types, crawl rules,authentication, and federation.
Plan crawl rules to optimize crawls
Crawl rules apply to all content sources in the Search service application. You can apply crawl rules to a particular URL or set of URLs to do the following things:
Avoid crawling irrelevant content by excluding one or more URLs. This also helps reduce the use of server resources and network traffic.
Crawl links on the URL without crawling the URL itself. This option is useful for sites that have links of relevant content when the page that contains the links does not contain relevant information.
Enable complex URLs to be crawled. This option directs the system to crawl URLs that contain a query parameter specified with a question mark. Depending upon the site, these URLs might not include relevant content. Because complex URLs can often redirect to irrelevant sites, it is a good idea to enable this option on only sites where you know that the content available from complex URLs is relevant.
Enable content on SharePoint sites to be crawled as HTTP pages. This option enables the system to crawl SharePoint sites that are behind a firewall or in scenarios in which the site being crawled restricts access to the Web service that is used by the crawler.
Specify whether to use the default content access account, a different content access account, or aclient certificate for crawling the specified URL.
Reference:https://technet.microsoft.com/en-us/library/jj219577.aspx