Shared parameters are parameter definitions that can be used in multiple families or projects.
Shared parameters are definitions of parameters that you can add to families or projects. Shared parameter definitions are stored in a file independent of any family file or Revit project; this allows you to access the file from different families or projects. The shared parameter is a definition of a container for information that can be used in multiple families or projects. The information defined in one family or project using the shared parameter is not automatically applied to another family or project using the same shared parameter.
In order for information in a parameter to be used in a tag, it must be a shared parameter. Shared parameters are also useful when you want to create a schedule that displays various family categories; without a shared parameter, you cannot do this. If you create a shared parameter and add it to the desired family categories, you can then create a schedule with these categories. This is called creating a multi-category schedule in Revit.