Shared files exist in two or more folders concurrently. Shared files are marked with a yellow plus sign in the icon in the Explorer view. For example, 
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Checking out a shared file from any location checks out all instances and checking in the file saves modifications to the vault and to all instances of the file. Only one user at a time can check out a shared file.
If the source file being shared is checked out, the new instance remains checked out and in the same state as the original.
Use the shortcut menu options Paste Shared and Share Here to share files.