Built-in identifiers are automatically declared by the compiler. Unlike reserved words, you can declare your own identifiers with the same spelling as built-in identifiers, if you do then your declaration will override the declaration made by the compiler. So for example the following declaration
type boolean = (no, yes);
will override the declaration created by the compiler
type boolean = (false, true);
Some built-in identifiers are used by extensions to Standard Pascal and are therefore only declared by the compiler when the extensions are enabled.