Here's what Ngaire says:

They just are asking that it be added to the booking. If you do not add it you are still allowed to participate but they really want to know the numbers so they are requesting if you intend to participate to add it to your booking.

Thanks, Ngaire!

Wendy
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Booked: Uniworld Egypt, 2015
Tauck Amsterdam to Budapest, 2016
Paul Gauguin, Societies & Tuamotus 2016