Target Plug-ins
The Plug-ins tab shows all the plug-ins found in the specified target location. Only plug-ins that are checked on the tab constitute the target content. Unchecked plug-ins are ignored by PDE. By default, all plug-ins in the target are checked.
If, for example, you are doing RCP development, it would be a good idea to select only the subset of plug-ins that make up the RCP feature. This way, other plug-ins in the target will not be part of your target and you will not accidentally reference plug-ins from outside the RCP subset.
The Plug-ins tab has several buttons to help with the selection of plug-ins, including the Add Required Plug-ins button. When this button is pressed, PDE scans all the checked plug-ins in the table, recursively goes through their dependencies and checks any dependency that is not already checked.