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There are many ways to single-source content in Flare, and at many different levels. One of the most powerful ways to take advantage of single-sourcing is to do so at the broadest level—across multiple projects. It's called "Global Project Linking."
If you have multiple Flare projects, you may find that you use the same content or files in some of those projects. Maybe there are a number of identical topics that you add to each project. Perhaps you have created a stylesheet that you copy and re-use in each project. Rather than creating this information separately, or relying on copy-and-paste actions, you can save time and ensure consistency by maintaining that information in just one project. When you want to use a file in another project, you simply import it by using a project import file. This creates a link between the source project file and the imported file in the secondary project. In the future, when you need to make changes to the file, you do not need to do so in multiple places, but rather in just one place—in the source project.
If you decide to take advantage of Global Project Linking, it is highly recommended that you use condition tags when doing so. By applying condition tags to your files (e.g., topics, images, stylesheets, page layouts), you can more easily control which files are imported into (or excluded from) a particular project. This is especially true if you elect to use the "auto-reimport" feature, which automatically imports files that you specify when you generate targets.
See Global Project Linking—Importing Files from Other Projects and About Condition Tags.