A partner code is like a "bonus" code provided by... a partner ;-)
With this code, it is possible to obtain a special boat, options, or a pack of options, depending on the case.
Using this partner code also gives you access to a special ranking in the race, where everyone who entered this code appears.
There are public partnerships, with codes more or less widely disseminated by the partner, and "private" partnerships with codes issued individually by the partner.
This code can be entered when registering for a race in the "Partner Code" field:
If you are already past that step, you can still enter your partner code later in the race, from the game screen menu: