Resources have properties that can be used to store meta-information about the resource. Your plug-in can use these properties to hold information about a resource that is specific to your purpose. Resource properties are declared, accessed, and maintained by various plug-ins, and are not interpreted by the platform. When a resource is deleted from the workspace, its properties are also deleted.
There are two kinds of resource properties:
If you follow the convention of qualifying property key names with the unique id of your plug-in, you won't have to worry about your property names colliding with those of other plug-ins.
If your plug-in needs to store persistent information about a project that is much larger than 2 KB, then these properties should be exposed as resources in their own right, rather than using the persistent properties API.
See IResource for a description of the API for getting and setting the different kinds of resource properties.