Third-party protection (Swedish: sakrättsligt skydd), sometimes called perfection, means that a right to property applies not only between the parties but also against third parties, such as creditors. For shares, this matters especially in a pledge and a transfer.
If there are share certificates, a pledge is normally protected through delivery, meaning the share certificate is handed over to the pledgee. If there is no certificate, it is instead achieved through notice to the company (denuntiation). The pledge is then noted in the share ledger, giving traceability.

A notice of pledge (denuntiation) is a notice that gives the pledgee protection against third parties.
Read more
A pledge of shares means shares are provided as security for a debt, often a loan.
Read more
Possession means someone actually holds and can dispose of an item, whether or not they have a right to it.
Read more