Helium 10 uses the term Search Volume to refer to what Amazon calls Search Frequency Rank. Some people call this SFR in seller jargon.
Amazon defines Search Frequency Rank as "the numeric rank of a specific search term's popularity when compared to all other search terms during a given period of time." Amazon limits reports of search frequency rank (located in its Brand Analytics) to its Brand Registered Sellers only.
Helium 10 members can use Magnet and/or Cerebro to find a search term's Search Volume, which is sourced directly from Amazon.