During the prosecution of a patent application, various principles are employed to assess whether an application meets the criteria for allowance, and the principle of unity is one of them. 

The principle of unity, a fundamental principle in patent applications, means that an invention or utility model patent application can only contain one invention or utility model. If a patent application includes multiple inventions or utility models, it is patentable only if these inventions or utility models share the same or corresponding special technical features. 

Its legal basis is derived from Article 31 of the Patent Law of the People's Republic of China: 

A patent application for an invention or utility model shall be limited to one invention or utility model. Two or more inventions or utility models belonging to a single general inventive concept may be filed as one application. A patent application for a design shall be limited to one design. Two or more similar designs for the same product, or two or more designs that are incorporated into products belonging to the same category and are sold or used in sets, may be filed as one application. 

Why is such a rule set during the patent application process? 

1. To prevent the integration of multiple irrelevant inventions into a single patent application, which would allow applicants to pay only one application fee while receiving protection for multiple different inventions.
2. To facilitate the classification, search, and examination of patent applications by examiners, thereby improving their work efficiency and accelerating the overall patent application process.

In what situations would a patent application not comply with the principle of unity?

Situations where a patent application suffers from lack of unity are listed below:

1. The original claims include more than one invention that does not comply with the unity requirement. 
2. The independent claims added or replaced in the amended application documents do not have unity with the inventions in the original claims. 
3. One of the independent claims lacks novelty or inventiveness, rendering the remaining claims lack of unity therebetween. 

When an examiner points out a deficiency of lack of unity in a submitted patent application, normally the examiner will reject the application and require the applicant to amend the original application and resubmit it. The applicant can choose to directly amend the application or file a divisional application to overcome the deficiency of lack of unity. 

Notably, a patent application may be rejected due to lack of unity. However, lack of unity is not a legal ground for invalidation against a granted patent.

To avoid rejection by the examiner due to lack of unity during the patent application process, an applicant can take the following steps to determine if their patent application meets the unity requirement:

Analyze the Relevance of Technical Features:

• Confirm if all the inventions in the claims share a common technical feature, which should be a key part of solving the technical problem.
• Determine if these technical features have the same function and effect in all the claims.

Ensure Consistency of the Invention Concept:

• Verify if all the claims revolve around a single inventive concept or aim to achieve the same technical objective.
• Ensure that the inventions in the claims are technically related rather than being independent, unrelated inventions.

Check for Common Technical Means:

Confirm if the technical features in the claims are common technical means in the art. If they are, these features should not be considered as forming the basis of unity.

Review the Hierarchical Structure of Claims:

• Ensure there is a clear technical relevance between independent and dependent claims. Dependent claims should further limit the technical features of the independent claims.

Refer to Examination Guidelines and Cases:

• Refer to the specific regulations and interpretations of the unity principle in the national or regional patent examination guidelines.
• Review relevant examination cases to understand how examiners examine the unity of invention in practice.

Practical Steps:

1.    Identify Common Technical Features:
o    List the technical features of each claim and find their common points.
o    For example, if the application includes a device and a method of using that device, confirm whether the technical features of the device and the method are consistent.

2.    Analyze Technical Effects:
o    Confirm if these technical features achieve the same or similar technical effects.
o    For example, if one claim involves a new material and another claim involves a product made from that material, verify if the technical effects of the material and the product are consistent.

3.    Consult Professional Opinions:
o    In case of uncertainty, consult a patent agent or patent attorney. They have the experience and expertise to help determine if the patent application meets the unity requirement.

By following these steps, an applicant can have a better understanding when determining if their patent application complies with the unity of invention principle and avoid rejection due to unity issues.