[MSDN Library June 1998] Optional Features for Address Book Providers (100%)

MSDN Library Visual Studio 6.0 > Platform SDK > Database and Messaging Services > Messaging API (MAPI) > Guide > Developing an Address Book Provider

[MSDN Library June 1998] Optional Features Transport Providers Can Implement (80.1120448179272%)

MSDN Library Visual Studio 6.0 > Platform SDK > Database and Messaging Services > Messaging API (MAPI) > Guide > Developing a Transport Provider

[MSDN Library June 1998] Optional Features of ACM Drivers (25.1251571009636%)

MSDN Library Visual Studio 6.0 > DDK Documentation > Windows 95 Device Driver Kit > Design and Implementation Guide > Audio Compression Manager Drivers > About the Audio Compression Manager

[MSDN Library June 1998] Optional Report Message Properties (22.8956098249576%)

MSDN Library Visual Studio 6.0 > Platform SDK > Database and Messaging Services > Messaging API (MAPI) > Guide > MAPI Programming Basics > Messages > About Message Properties

[MSDN Library June 1998] Optional Interfaces for the Visual Designer (21.7449436528384%)

MSDN Library Visual Studio 6.0 > Tools and Technologies > ActiveX Designer Programmer's Reference > Chapter 3: Designing and Implementing an ActiveX Designer > Creating the Visual Designer