Importacular makes use of criteria sets to look up constituents and relationships for both individuals and organisations. How are participants looked up though as there are no participant criteria sets available.
The difference with participants is this. If you are creating a new constituent then Importacular will not look to see if the participant is in the event. If you are updating a constituent or Importacular has found a match with a constituent then it will look to see if that constituent is already a participant in the event. It is not possible to create a non-constituent participant for an event from a constituent.
A constituent can have a guest associated with them. Again if the guest is a constituent we look them up in the event. If the constituent id cannot be found in the event then we look them up by first name and surname.