Every item that can be shipped in the game has a item rank to it. The rank of the item will determine how much money you are going to receive for selling it, and what its STA/FUL recovery effectiveness is if the item is edible. The ranks are (from lowest to highest) D, C, B, A, and S.
Each dot-marker on the Quality, Size, and Freshness scales will build the item's rank. Different combinations of Qu, Sz, and Fr can result in the items having the same overall rank (A, B, etc.), but the profit earned will be different depending on which and how many dots are filled in.
If you stack items together in your Rucksack, the two item's ranks will average together. Putting together a B-Rank Turnip with your S-Rank Turnip will combine into a stack of two A-Rank Turnips.
Normally you won't know how much profit each Qu, Sz, and Fr scale is worth for a specific item, but an Egg's scale is very easy to know. If you have an S-Rank Egg (10 dots in Qu, 10 dots in Sz, and 10 dots in Fr) you will earn 300 G if you ship it. Each dot on the scales are worth 10 G.
So let's say you have 2 different B-Rank Eggs, a 10/4/10 and a 9/3.5/10:
- 10 Qu (100 G), 4 Sz (40 G), 10 Fr (100 G) = 240 G profit
- 9 Qu (90 G), 3 1/2 Sz (35 G), 10 Fr (100 G) = 225 G profit
Most of the scales will not be this simple. There might be an item that earns you 8 G for each dot of Size, but only 4 G for each dot of Quality! The calculations for shipping price are hidden from view so a player won't have any idea how much profit will be earn for each of the three different scales.
For crops, high ranks can be produced by using the minimum amount of Sun and Water points required for the plant to mature. This is difficult to do in an outdoor environment, sine you don't have any way of controlling the weather. You will have better success reaching S-Rank crop status if you use the protection of the indoor Greenhouse.
For animal products, the more hearts the creature has, the higher quality the item can be. If you want S-Rank products you'll have to play the animal touch screen mini games. The other way to earn S-Rank animal items is to have the animal win its Animal Festival. You'll earn a high-profit item from a wining creature without having to wait for the random mini games to appear each day.
For wild grown items like grasses and flowers, it seems the longer you leave it alone to grow, the higher the rank will be. If you're trying to earn an S-Rank Purple Grass and you're not sure if it's ready yet, save your game before you pick it up off the ground. If it's not an S-Rank then reload and leave it alone. The grass will be there until the end of the season.
Mine item ranks are random, but the deeper you go in the mine, the higher the rank will be.
Cooked Recipe item ranks will be calculated based on the combined ranks of the cooking ingredients.
The other option to raise item ranks is to use the Rank Maker machine in your Maker Barn. To use the machine, place an item in the machine and just leave it alone. It takes about a season for an item to increase a full rank.