You can create custom parameters for any element or component category in your project. Parameters that you create display in the Properties palette or Type Properties dialog under the group you define and with the values you define.
For example, if a parameter in a family or a project needs to be tagged, that parameter must exist in the project (or element family) and the tag family. Therefore, taggable parameters need to be shared parameters. Shared parameters can also be used when elements of 2 different families are scheduled together. For example, if you need to create 2 different Isolated Foundation families, and need the Thickness parameter of both families scheduled in the same column, Thickness needs to be a shared parameter that is loaded in both Isolated Foundation families.
Project parameters are specific to a single project file. Information stored in project parameters cannot be shared with other projects. A project parameter can be used, for example, to categorize views within a project.