How to use descriptors as parameters?

The base classes provide and implement the constant and modifiable descriptor operations regardless of the actual type of the derived descriptor. For consistency, they (basic types) should be used as arguments to functions, allowing descriptors to be passed without restricting the caller of the function to using a specific type.

For example, the calling function can call a function with anything derived from TDesC and TDes. For example an HBufC and a TBuf<8>, or a TPtr and a TBuf<3>:

void SomeFunction(const TDesC& aReadOnlyDescriptor, TDes& aReadWriteDescriptor);

further reading http://descriptors.blogspot.com/