A "supplementary patent application" (also referred to as a "top-up patent application") is a term of art used by patent attorneys to refer to a second or subsequent patent application filed during the first year of the patent application procedure which includes new subject matter not found in a first patent application concerning the same topic, with the intention that a later patent application proceeds to grant including priority claims from both the first and any supplementary patent applications.
Once a patent application has been filed, it is not possible to add subject matter to it. Any amendments must be based on subject matter which is already included in the application. Subject matter can usually also be deleted. However, it is very common for applicants to develop their invention shortly after the filing of their first patent application and to wish to include new subject matter. As they cannot add it to a pending application, the patent application procedure provides a mechanism for them to file a new application including the additional subject matter and for the applications to be combined into a single application in respect of any given territory for future prosecution.
When there is a desire to obtain a priority date for subject matter which is not found in a first application, the procedure which is often appropriate in Europe is for a supplementary patent application to be filed. At the end of the first year of the patent application process, one or more later applications (e.g. international applications) are filed which claim priority from the first application and also from any supplementary applications which have been filed in the intervening time. These later applications proceed through examination.
Importantly, where this occurs, the later application(s) has multiple priority dates. The priority date of any given claim of the later application(s) is the date of the first of the various applications (the first application, the supplementary application(s) or the later application(s)) to contain the subect matter of the claim. The priority date is the date at which the claimed subject matter must be new and inventive*. If subject matter was added for the first time to the later application(s) then the filing date of the later application is the relevant date for assessing the patentability of the later application(s). This means that patent applications can have claims with different priority dates.
The practical effect of this is that, although subject matter cannot be added to a patent application, supplementary applications can be used to obtain priority dates for new subject matter during the first year of the patent application procedure, without a requirement to prosecute multiple separate patent applications to grant.
Whether a supplementary patent application does, or does not, claim priority from the first application is a question to consider on a case by case basis. A downside of claiming priority from the first application is that doing so irrevocably loses the possibility of rejuvenating the priority date by withdrawing the various applications leaving no rights outstanding and without any application being published or serving as the basis of a priority claim.
The possibility of filing supplementary or later patent applications with additional subject matter not included in a first patent application is one of the reasons why it is often preferable to avoid disclosing the content of a patent application even after a first patent application has been filed.
* In Europe, it may be sufficient to be novel but not necessarily inventive over earlier filed patent applications which were not yet published at the priority date.