$IF

// pointermath directive did not exist prior FPC 3.0.0
{$if FPC_VERSION > 2}
    // pointer arithmetics is bad. very bad
    {$pointermath off}
{$endif}

//Befor you need write these conditions to check some conditions:
{$define SOMETHING}
{$define SOMETHINGELSE}
{$ifdef SOMETHING}//Union $IfDef to check multiple conditions
    {$ifdef SOMETHINGELSE}
   {$ModeSwitch advancedrecords}
  {$endif}
{$endif}

//But with the {$IF} you can check conditions together:
{$if defined(SOMETHING) and defined(SOMETHINGELSE)}//simple and readabl instead of union {$IFDef}`s
  {$ModeSwitch advancedrecords}
{$endif} 

{$if defined(somthing) or defined(somethingelse)}
  //Whatever you need!
{$endif}

{$if undefined(what) and defined(somethingelse)}
  //Just for note, Another usage!
{$endif}