By default, all projects are completely self-contained; artifacts in one project can only be linked or associated with artifacts in the same project. However, there is a way for the project admin to selectively allow other projects to access artifacts in the current project. (Administration -> Project Associations)
The cross-project functionality is being introduced gradually. As of Spira version 5.4, it applies only to requirements, test cases, and incidents.
This article explains how and where the artifacts from another project can be used.