What is experiment confirmation?
Game of Whales is able to send to the game side the data about the experiment which the player was included in, the experiment group and the group payload. But the game side is responsible for the correct handling of this data.
So after the applying rules from the received payload or necessary settings (if you don't use payload) the experiment confirmation should be sent to Game of Whales. This will mean that the data that the GOW will receive from this player will reflect the changes that you want to check during the experiment.
There is a special method to send the confirmation in GOW SDK. More details can be found in the implementation guide:
If you use REST API, you should send expId argument in each player's request to GOW server as long as the experiment is active.
The part of the confirmed experiment audience is shown in the experiment report.

A low percentage of confirmed audience may mean that the experiment settings were not applied on the game side and the statistics collected during the experiment are not reliable.