It’s important to remember that spaces give you access control not confidentiality. The data in a space is readable by any user or application with access to that space, it’s not encrypted.
As the next sentence says "Spaces are a very flexible primitive, and the range of uses is deliberately broad." Access control by groups fits a wide range of applications. Encryption with multiple access is a specialized need that might be built on the primitive.
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As the next sentence says "Spaces are a very flexible primitive, and the range of uses is deliberately broad." Access control by groups fits a wide range of applications. Encryption with multiple access is a specialized need that might be built on the primitive.