We store booker and traveller information in two ways: ID and email.
booker_id gives the booker's unique identifier (ID) which will never change
booker_email is the booker's email address at the time of that booking
traveller_id is the traveller's unique ID
traveller_email is the traveller's email address at the time of booking
If the Booker or Traveller later change their email, future bookings will have the new email, but past bookings will keep the old one.
If you have an old custom report template file, it may still have the ID column when you would prefer the email. You can correct this by updating the template file.
In the RawData tab:
replace booker_id with booker_email
replace traveller_id with traveller_email
If you want to have both ID and email, you can insert new columns instead.