There are many mods that adds item filter (or fluid filter) items, but these filters are only recognized in their own mods. There should be a way to let other mods know if an item is a filter and how it filters item. Maybe like ItemLookupApi<Predicate<ItemStack>,Void>.

Can you explain what you mean by filter? Is it prevent putting items into certain slots? If so couldn't a tag be used?

Item filter item. An item which stores the filter info. It can't be a simple tag because it's dynamic and item-specific, each item can have different filter info.

For example, there is a pipe block which accepts an item filter. You can directly set the filter to one single item, in this case you can just use the item itself as filter. But if you want more complex control, you have to use some expert way to set the filter. The most common way is that the mod provides an item which has GUI for player to set the filter. What I mean is this item filter item.

Ah ok, yes I see now. Thanks. I didnt catch onto the fact that its something that a user configures.