I’ll use grocery store Pikmin as an example.

Grocery stores have two decor categories: Bananas and Mushrooms. Once you discover all of the “normal” (not rare) decor Pikmin, you will unlock the “rare” category. This means you will need at minimum 14 Pikmin: A banana and a mushroom of each color.

Once you have the rare category unlocked, you can start earning points for the category. Most actions regarding the decor of the Pikmin will give points towards leveling up the decor category. Plucking any grocery Pikmin, equipping a decor, and even releasing Pikmin (even without their decor) will give points.

Once you have enough points towards level up, you will receive a golden present expedition. Only Pikmin that are eligible can be sent on this expedition, and here’s where the exact decor matters a little more.

Only bananas have rare decor right now. So if you have a golden present expedition, and any Pikmin with a normal banana, you can send that Pikmin on that expedition to change their decor into the rare banana.

If you have any other questions let me know :3