Base level indicates the support level of the oldest Compiler.
When the Compiler calls the preprocessor, response-code-2 is used to inform the preprocessor of its support level. When the preprocessor returns to the Compiler, it uses the same parameter for its support level.
As features are added, the support levels are incremented; each level includes the support in the levels before it. To ensure that old Compilers are supported, the value 8224 is a special case: it indicates that the parameter was not initialized and represents the "base level". Similarly, an old preprocessor that does not set a support level does not change the parameter value, so all values of 32767 or lower are treated as the "base level".
Features not in the base level are highlighted with a warning.