Design-time packages are used to install components on the IDE's Tool palette and to create special property editors for custom components. Which ones are installed depends on which edition of Delphi you are using and whether or not you have customized it. You can view a list of what packages are installed on your system by choosing Component > Install Packages... .
The design-time packages work by calling runtime packages, which they reference in their Requires clause. For example, dclstd references vcl. The dclstd itself contains additional functionality that makes many of the standard components available on the Tool palette.
In addition to preinstalled packages, you can install your own component packages, or component packages from third-party developers, in the IDE. The dclusr design-time package is provided as a default container for new components. See Installing Component Packages.
C++Builder supports compiling design-time packages that contain Delphi source files. However, if any of those Delphi sources make reference to IDE-supplied design-time units such as DesignIntf, DesignEditors, and ToolsAPI that exist in DesignIDE100.bpl, you must take steps to ensure that the references can be resolved by the C++Builder package. See Compiling C++ Design-Time Packages That Contain Delphi Source for details.