User interface > Translation options > Input options > Definitions
Delphi code often contains directives for conditional compilation of parts of the source text. Delphi2Cpp evaluates such directives too. You can set the definitions in the option dialog
There are limitations for the evaluation of such expressions.
If code of the Delphi RTL shall be translated, it is recommended to set PUREPASCAL defined, to avoid problems with inline assembler code.
Incomplete definition can lead to hard to find bugs, as for example in System.Windows.pas