Libraries and frameworks are designated as Required by default, but you can change this designation to Optional.
Required libraries must be present on the host computer for the product to load.
Optional libraries do not have to be present for the product to load.
A Required framework will always be loaded into memory, but an Optional framework will be loaded only if needed.
The initial load of the application will be faster if a large library that is never needed is designated as Optional.
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原文：http://www.cnblogs.com/ioriwellings/p/3794803.html