where X is replaced by 0, 1, 2, 3, etc. for each photographer and submitter.
Always populate Encounter.submitterID. This assigns the Encounter to a user account (specifically that username) that will then own it (view/edit), but it makes no assumption about the submitter and photographer Contact data or their emails addresses or names. It is an entirely independent field.
For Contact info (submitter and photographer info), if that User account already exists by a unique email address, only the Encounter.submitterX.emailAddress and Encounter.photographerX.emailAddress fields are considered. This prevents a bulk import from overwriting personal information that a User has manually entered for their user account, such as preventing a misspelling of their name in a bulk import sheet from changing the correct spelling the user has already set.
If the User account does not already exist (via a unique email address look up), then the other submitterX and photographerX columns are considered when creating the new User in the database. In subsequent bulk imports then, those same fields will be ignored.