A client-side developer has easy access to tracking items that enter the player inventory, but tracking items that enter the sacks require the player to have sack messages on, and the developer to be reading into those messages with hover tooltips:
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Being able to read an item directly instead of text parsing would be pretty convenient and let developers more easily expand upon what their features can be scoped to support.

What kind of features could use this info?

• Loot trackers
  • SBA's item tracker is a prime example of something already in the game, but there's also other cool stuff you could do. One idea that comes to mind is allowing the user to create a HUD that allows them to track their progress towards a set goal for a particular item (iron man players/streamers might find this particularly useful)
• Profit trackers

What about chat messages/tablist widgets?
This or other meta ways of getting that information are currently possible, but tend to be complex or generally imperfect.

One arbitrary example - if you want to track collection for a garden crop, you can use the collection milestone tablist widget.
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However, when you hit max milestone you could no longer do this. Same goes for the collection widget on the tab list:
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