The version number in applets, scripts and packages specifies the Curl API that has to be supported by the Curl Engine, i.e. by the plug-in. It is possible to have several APIs installed at the same time. Currently, Curl Inc. offers versions 5.0.3, 4.0.5 and 3.0.11 for download.
Within an application the Version number cannot be changed, i.e. each package has to be written for the same API. This eliminates the problems with downwards-compatibility.
However, it is possible to use other version by creating sub-applications, that are applications which run within an application. There is a (limited) possibility to exchange information between the main-applet and sub-applets.