A partner code is like a "bonus" code provided by... a partner ;-)
With this code, it is possible to obtain a special boat, or options, and in any case, to participate in the race by also appearing in a special ranking dedicated to this partnership.
There are public partnerships, with codes more or less widely disseminated by the partner, and "private" partnerships with codes issued individually by the partner.
For example, in the case of a partnership with a newspaper, the code will be distributed by the newspaper to its readership. The game will also be available on the newspaper's website pages. When you register on this dedicated interface, the code can be pre-filled. It can be deleted if you don't want to benefit from it (using the code is optional).
With this code, you enter a dedicated ranking of virtual navigators who are also readers of this newspaper, and you will also be able to participate in the race with a boat designed for this newspaper. In addition to the prizes offered on the "standard" virtual race, you then ALSO have the chance to win prizes offered by the partner according to their conditions. It's like a race within the race, and that's one of the reasons why once you have entered a partner code, you can't remove it.
This code can be entered upon registering for a race, in the "Partner Code" field.