The “to System Folder" contains libraries, extensions, control panels, control strip modules, and scripting additions for the security toolkit.
The “Applications" folder contains the Signing and Verifying applications. The Signing Application is only available in the SDK. It is intended to be used by developers to produced signed files for distribution over the internet. Files signed with the Apple Signer will automatically be verified by any applications using URLaccess.
The “Libraries" folder has stub libraries to link against when building your own programs. The KeychainLib, CryptoMessageSyntaxLib and FileSigningLib libraries are contained in the Security Library in the actual software product.
The “Includes" folder has the header files needed to create applications and libraries using the Security toolkit.
“Utilties" contains additional programs to help with exploring the Security Toolkit.
“Documents" contains various documents discussing how to use the Security Library and the various utilities.
The Keychain and Message Signing APIs are still in Alpha stage. They may change slightly for the final release.
The Cryptographic Message Syntax APIs have not been qualified for the 8.x release. The APIs may change before they are released with an OS version. The libraries are useful for working with encrypted/signed message formats and have been left in the SDK for exploratory use.