A custom package is either a .bpl you code and compile yourself or an existing package from a third-party vendor. To use a custom runtime package with an application, choose ProjectOptions and add the name of the package to the Runtime Packages edit box on the Packages page. 

For example, suppose you have a statistical package called stats.bpl. To use it in an application, the line you enter in the Runtime Packages edit box might look like this:

vcl;rtl;vcldb;stats //In VCL applications

If you create your own packages, add them to the list as needed.