A company registration number is a unique identifier that Bolagsverket or the Swedish Tax Agency assigns to a limited company or association. It is used to identify the company in all official contexts.
In the share ledger, the registration number is given for shareholders that are legal entities, just as a personal identity number is given for natural persons. It is part of the legal requirements for what the share ledger must contain.

A legal entity is an organisation that can have its own rights and obligations, such as a limited company.
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A limited company is a business form that is its own legal entity and is owned through shares.
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Bolagsverket is the Swedish authority that registers companies and company information.
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A natural person is an individual human being, as opposed to a legal entity such as a company.
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