An alternative is someone who stands in for an indigenous person at a ceremony when no indigenous people are available to attend the event.
The reasons for requiring indigenous people at an event are varied. For example, the event might affect their interests and rights or a politician might think that an indigenous person or two would make a good backdrop for a photo op at the event.
The reasons why an indigenous person might not be available to attend the event are also varied. For example, all indigenous people might have something better to do at that time—which, depending on the nature of the event, might be just about anything—or there might be no indigenous people who feel inclined to serve as a backdrop for a politician at that time.
Because there are so many times when it is appropriate to have indigenous people at events, but there are also many time when none are available or none are willing to make themselves available for a cheap political publicity stunt, alternatives serve a very important purpose.