The opt-out is the option, available during the transitional period, to remove a classic European patent from the jurisdiction of the UPC.
Who, What
The proprietor (or the applicant) may exclude from the jurisdiction of the UPC: a classic European patent, a European patent application, or an SPC based on such a patent (Article 83(3) of the Agreement on a Unified Patent Court and Rule 5.1 of the Rules of Procedure). Caution: where there are multiple proprietors (including different proprietors in different countries), all must act jointly; an opt-out filed by only one is ineffective.
Recall: the unitary patent is not affected. An opt-out cannot be filed for a unitary patent; choosing unitary effect means accepting the UPC for good.
How
The opt-out is declared to the Registry of the UPC, electronically, and is free of charge. It takes effect upon its entry in the register (Rule 5 of the Rules of Procedure).
Effects and Limitation
Once the opt-out is recorded, the patent falls under the jurisdiction of national courts only, and this applies for the entire lifetime of the patent. However, the opt-out can no longer be exercised if an action has already been brought before the UPC for that patent: first come, first served.