The purpose of this site is to help players prioritize their item enhancers for the Pokémon they plan to play. Items that are useful on many Pokémon are given higher weighting than ones who only have a few Pokémon using them. The weighting assumes that Pokémon will be played roughly equally, so bear in mind your preferences when deciding how to use your item enhancers.
Data for recommended builds taken from www.unite-db.com. Each Pokémon's build is weighted by the total number of builds available for that Pokémon; this is to prevent Pokémon with a large number of sets from skewing the weight in their favor. These weights are aggregated across all the sets for all the chosen Pokémon, and sorted for display in a table. The more sets that use an item, the higher its weight and by extension the more highly it should be prioritized.
For example, suppose I want to play Blastoise, Venusaur, and Eldegoss, who have the recommended held items listed below.
The weight calculations are described below.
Without getting too bogged down by the numbers, it should be apparent that Buddy Barrier and Muscle Band are integral to all three of the Pokémon I want to play (every set for every pokemon uses them), and should be prioritized before any other items. From here, the prioriziation is a bit flexible since there are no stand-out items (the weights and number of Pokémon they come from aren't all that different).
Once again, it helps to bear in mind this weighting assumes the player will play the selected Pokémon roughly equally; if I were plannig to play Eldegoss 90% of the time I should prioritize the items that Eldegoss benefits from, even if they have lower overall weight.