The scenario: you have a large deck, and a second deck of material you already know cold. You want the large deck minus everything the small deck covers. Or you downloaded a comprehensive shared deck and want to strip out the cards your own deck already handles, keeping only the new material.
This is subtraction: deck A minus deck B, keep what is unique to A.
The manual way
1. Import deck B into a staging deck. You need both decks in the same profile for any comparison to work, so import B alongside A.
2. Run Find Duplicates. In the browser, Edit > Find Duplicates flags exact first-field matches. Select the matches that belong to deck A and delete or suspend them.
3. Repeat until your eyes blur. Find Duplicates only catches exact text matches. Every reworded duplicate survives, so you go back through the browser searching key terms by hand, card by card, deciding whether each one is covered by deck B.
4. Delete the staging deck. Clean up the import so it does not pollute future duplicate searches.
The failure modes are nasty. Batch-deleting on exact matches is destructive: one wrong selection and you nuke cards with months of scheduling history. The manual search pass is where most people quit, which leaves the reworded duplicates in place — the exact cards that silently inflate daily reviews. And direction matters: A minus B is a completely different result from B minus A, and nothing in the manual workflow stops you from deleting from the wrong deck.
The better way: subtraction as an operation
Upload both .apkg files and pick Subtract, then choose the direction: keep cards unique to deck A, or unique to deck B. The matching is semantic, so a card in A gets excluded when deck B covers the same fact, even if the wording is completely different. Conflicts get flagged for review instead of being silently deleted. You download the result as one .apkg.
What you skip: the staging import, the exact-match batch delete, the card-by-card search pass, and the risk of deleting the wrong deck's cards. The result is the deck you would have built manually if matching by meaning were as cheap as matching by text.
The common uses
Trimming a shared deck. Download AnKing, subtract your own cards, study only what is new to you.
Building a weaknesses deck. Take your full deck, subtract the subdeck you consistently score well on, and drill the remainder.
Splitting work with a partner. Each of you makes cards for half the lectures, then subtracts the other's deck to verify there is no duplicated effort before merging.