There are three different classifications a Block Producer can fall into: 1) The Top 21 who produce and validate blocks, and are paid a portion of block rewards and vote-based rewards, 2) A Producer whose ranking is below 21, but receives enough votes to receive at least 100 EOS of vote-based reward within a given day, 3) All other Block Producer Candidates who have registered their account as a Candidate through the regproducer contract.
The term ‘Standby Producer’ refers to all Producers who fall into the second classification above.