K13 is a form from Skatteverket that is attached to the income tax return. It is used by shareholders in close companies who have disposed of qualified shares through a share exchange (Swedish: andelsbyte), and in certain similar situations.
Completing the form requires information on which shares were exchanged, when it happened and at what value. A share ledger with a complete history makes it easier to gather this information. Current rules and instructions are available from Skatteverket.

Qualified shares are shares in close companies covered by the so-called 3:12 rules.
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A close company is a company in which four or fewer owners together hold more than half of the votes.
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A share exchange means shares in one company are exchanged for shares in another company instead of cash.
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The threshold amount sets how much dividend or gain in a close company is taxed as capital income.
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