Bidders represent people that are attending your event or participating online in some way.
For most events you can think of bidders and guests as being interchangeable.
If you are selling tickets for your event then we recommend you have one bidder per guest. This lets you get an accurate count of who is coming and is required if you want to track per-guest details like meal choices or seating.
If you have an event that is just an auction and doesn't require tracking guests precisely then you can create a single bidder record to represent, for example, a couple rather than creating two bidders that share a bidder#.
Bidders that share a bidder number are said to be linked, see Sharing Bidder Numbers for more details on how this works.
There is an Additional Bidders field on the bidder record that can be used to track the names of other people bidding under this bidder. This field is purely informational and will be shown on the bidder's statement.
If you are using tickets and creating one bidder per-guest then this field is not used.