A PDE project is a project that "knows" it hosts a plug-in and is eligible for plug-in-specific operations.
PDE is fully capable of working on plug-ins in the workspace represented as ordinary projects. However, it cannot offer incremental file checking, capability-based filtering and other similar features if a project does not possess full PDE capabilities.
It is possible to convert a regular project into a PDE project at any point. For example, you may have some Java classes that you want to package into a library and share with others as a plug-in. Alternatively, you may want to get full support for manifest syntax checking and reporting that only PDE projects have.
To convert projects into PDE projects, select a project in the Navigator view. Select PDE Tools > Convert Projects to Plug-in Projects... from the context menu.
The wizard will list all projects that do not have PDE capability. Candidate projects do not need to have manifest files. If they are missing, PDE will create generic ones you can use as a starting point. Files that already exist will be left intact. Select one or more projects, click Finish.