An applicant can file a patent application either with provisional specification or with complete specification. In case the application filed with provisional specification, then the complete specification should be filed within the 12 months from the date of filing of the provisional application (Note: there is no further extension of time to file complete specification after expiry of aforementioned period).
At Indian Patent Office following types of patent applications can be filed depending upon the purpose: People should hire an experienced patent lawyers in delhi
Provisional Application
Indian Patent Law follows first to file system. A provisional application is an application which can be filed if the invention is still under the experimentation stage. Filing of a provisional application provides the advantage to the inventor in establishing the Priority date of the invention.

Ordinary Application
A patent application without claiming any priority either in a convention country or without any reference to any other earlier application filed in the Patent Office is known as ordinary application.

Convention Application
PCT International Application
A patent application filed in India as Receiving Office (RO) under Patent Cooperation Treaty (PCT) is an International application which can be filed in more than 150 countries by a single application.
PCT National Phase Application
When an international application is made according to PCT designating India, an applicant can file the national phase application in India within the period of 31 months from the date of international filing or the priority date, which is earlier.
Patent of Addition (Patent Registration in delhi)
When an invention is a slight modification of the earlier invention for which application is already filed or patent granted, the applicant can file the patent of addition application if the modification in the invention is new. One of the benefits of filing of patent of addition application is that there no need to pay separate renewal fee for the patent of addition during the term of main patent and it expires along with the main patent.
Divisional Application
This type of application, divided out the parent one, is known as the Divisional Application. The priority of all the divisional applications will be same as the priority of the parent application(Anti-dating).