Indexing is simply the work the Search Engine Robots do to give you information on discount travel agency. How does an indexing robot decide what to index? Well they just send their Bots or Spiders to crawl the web looking for discount travel agency on web sites.
If an indexing robot finds a document that contains discount travel agency, it may decide to parse it, and insert it into its database. How this is done depends on the robot. Some robots index the HTML Titles that contain discount travel agency and keep them in their database. Others will look for discount travel agency in the first few paragraphs, or parse the entire HTML and index all words.
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