A more thorough explanation:

The doctrine of continuity is a principle that applies to international law and patents.

In international law, the doctrine of continuity states that upheavals and revolutions within a country, changes in governmental forms, the extent of a country's territory, and measures taken during a military occupation do not affect the existence of the country. Therefore, these changes cannot lead to the extinction of the country.

In patents, the doctrine of continuity is the rule that a continuation or divisional patent application carries the effective filing date of its parent application if:

1. The parent application fully discloses the same invention
2. There is at least one common inventor
3. The parent application was still pending when the latter application was filed

A continuation-in-part application carries the effective filing date for everything disclosed in the parent application, but not for new material.

For example, if a patent application is filed on January 1, 2020, and a continuation application is filed on January 1, 2021, the continuation application will have the same effective filing date as the parent application. This means that any prior art that was published between January 1, 2020, and January 1, 2021, will be considered when determining the patentability of the invention.

The doctrine of continuity ensures that patent applicants have the opportunity to protect their inventions even if they need to file multiple applications to fully disclose the invention.