This command is usually performed without a project, as that is what you are checking out, so you need to specify the SCM URL. The URL takes the same format as the <repository><connection> element in your project.xml file.

This command is always performed on an existing project. It is used to update to the latest code from the server for the project.

This command is always performed on an existing project. It will apply a tag (or equivalent operation in your SCM) to the current checkout. You must specify a tag name.