Xcode add dependencies to "Project" OR "Target" (difference?)

Question

What's the difference between project dependencies and target dependencies?

• Project dependencies can be added here:

• Target dependencies can be added here:

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• in the case of external dependencies (via URL), do we need to add it to both project and target, or is one of those enough?
• in the case of local dependencies (via the workspace), do we need to add it to both project and target, or is one of those enough?