unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type 
  TForm1 =class (TForm)
    procedure FormCreate (Sender: TObject);
  end ;

var
  Form1: TForm1;

implementation

{$ R * .dfm}

// For example, the following four custom functions *****************************
function MyAdd (const a, b: Integer): Integer;
begin
  Result: = a + b;
end ;

function MyDec (const a, b: Integer): Integer;
begin
  Result: = a-b;
end ;

function MyMul (const a, b: Integer): Integer;
begin
  Result: = a * b;
end ;

function MyDiv (const a, b: Integer): Integer;
begin 
  Result: = adiv b;
end ;
 // ********************************************** ******

// call test
procedure TForm1.FormCreate (Sender: TObject);
const 
  x =8;
  y = 2;
begin 
  ShowMessageFmt ('% d,% d,% d,% d', [MyAdd (x, y), MyDec (x, y), MyMul (x, y), MyDiv (x, y)]);
   {Display result: 10,6,16,4}
end ;

end .

We can cut and save the custom function (or other code) in a text file (for example: C: \ DelphiFun \ MyFun.inc);
then use {$ INCLUDE C: \ DelphiFun \ MyFun.inc} or {$ IC: \ DelphiFun \ MyFun.inc} can be imported again (relative paths can be used).
Here is the code after use:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type 
  TForm1 =class (TForm)
    procedure FormCreate (Sender: TObject);
  end ;

var
  Form1: TForm1;

implementation

{$ R * .dfm}

{$ IC: \ DelphiFun \ MyFun.inc}

// call test
procedure TForm1.FormCreate (Sender: TObject);
const 
  x =8;
  y = 2;
begin 
  ShowMessageFmt ('% d,% d,% d,% d', [MyAdd (x, y), MyDec (x, y), MyMul (x, y), MyDiv (x, y)]);
   {Display result: 10,6,16,4}
end ;

end .

In addition: The C language obj file is completed with the {$ L path} instruction.