Understanding Code Rules - Form Transformation

1. Hello World in the first column is a comment in the CodeRules.txt file. You add comments to the CodeRules.txt file by using the <comment> special word and then writing your comment. Comments in the CodeRules.txt file are ignored when transforming code.
The example given is a little confusing because it also has a comment in columns 2 and 3 of the table, but these are unrelated to the <comment> in column 1.

2. !var! (or !var1! or !var2!, etc.) lets you specify a variable in a code rule. The <declareVariable> special word specifies that the transformed code must declare a variable. <declareVariableType> specifies the data type for this new variable. You use !declaredVariable! in the code rule where you want the transformed code to use the new variable that was declared.

The example given is confusing because as you mention, is assigns a Boolean value to an Integer. A better example would be this one, from the actual CodeRules.txt file:

<find>
!currForm!.!var1!.HEIGHT(!var2!)
<replace>
!currForm!.!var1!.HEIGHT := !var2!

<find>
CurrForm.!var1!.HEIGHT;
<declareVariable>
!var1!Height
<declareVariableType>
Integer
<replace>
!declaredVariable!;

Then if you have code like this:

OnRun=BEGIN
…
  MESSAGE('Hello');
  CurrForm.x.HEIGHT(NewHeight);
  MESSAGE('World');
END;

It would be transformed to this:

VAR
  xHeight : Integer;
OnRun=BEGIN
…
  MESSAGE('Hello');
  xHeight := NewHeight;
  MESSAGE('World');
END;

Hope that helps.
I have updated the Code Rules topic to try to make it clearer. The updated version will be available on MSDN (http://msdn.microsoft.com/en-us/library/dd338843.aspx) in a few months when we next update the Developer and IT Pro Help.

first of all: Thanks for your answer

No problem. :-)

<comment> in the first column specifies that the next line(s) of the CodeRules.txt file are comments. These lines don't map to anything in the code on the form. They are ignored by the transformation tool.
You could have something like this in the CodeRules.txt file:

<find>
x := z;
<comment>
This is a comment.
I can write notes here about what this rule does.
Until the transformation tool finds the next special word, all of these lines are parsed as comments. 
<replace>
;

The <comment> line and the next three lines are ignored. When the transformation tool gets to the <replace> line, it recognizes the <replace> special word and knows that the comments have ended.

The above is exactly the same rule as

<find>
x := z;
<replace>
;

In the example from your original post:

<find>
x := z;
<comment>
Hello world

The Hello world line does nothing.
Also, since there is no <replace> in that example, the found line is deleted.

Make sense?