Using Browse Nodes
Amazon.com (like all the Amazon sites) organizes it's web pages using "browse nodes." A browse node originally represented a specific page at Amazon.com, but is now used most often to create a category of related products.
A "browse node ID" is used to identify a specific browse node. The ID is an integer value. There's no pattern to these numbers other than that each browse node has a unique ID. Browse node IDs are used by the BrowseNodeLookup and ItemSearch operations of Amazon's E-Commerce Services.
You can find browse node IDs in the A2S documentation and by looking at URLs at Amazon.com.