PS3.19 of the DICOM Standard specifies an Application Programming Interface (API) to a DICOM based medical computing system into which programs written to that standardized interface can "plug-in" (see Figure 6.19-1). A Hosting System implementer only needs to create the standardized API once to support a wide variety of add-on Hosted Applications.

In the traditional "plug-in" model, the "plug-in" is dedicated to a particular host system (e.g., a web browsing program), and might not run under other host systems (e.g., other web browsing programs). PS3.19 defines an API that may be implemented by any Hosting System. A "plug-in" Hosted Application written to the API would be able run in any environment provided by a Hosting System that implements that API (see Figure 6.19-2).

PS3.19 specifies both the interactions and the Application Programming Interfaces (API) between Hosting Systems and Hosted Applications. PS3.19 also defines the data models that are used by the API.