Default Contact For

This field can be used to indicate that a particular contact is to be designated as a default contact for one of several purposes.

When specifying that a contact is a Default Contact For the purposes of Bill To, Ship To or Remit To, note that you can only have one contact for an organization designated as the default for each of these three options.  When designating a contact as a Default Contact For, any other contact currently designated as such will have that attribute removed as part of saving this record (to ensure that only one can have the designation).  

Note that the Bill To and Ship To contacts associated with a project invoice setup are those contacts associated with the project's 'project organization', while the Remit To contacts are those contacts associated with the project's 'owning organization'.