This means that a comment can include plain text, formulas, expressions, or even XML code as long as you know that that XML code will not be validated: it will ignored by the parser.

To create a comment, you use the following formula:

<!-- Blah Blah Blah ->

Between <!-- and -->, any text in that section is considered a comment and you can include anything you want. Both sections of the comment use two dashes, not more, not less. Here is an example:

<?xml version="1.0" encoding="utf-8"?>
<!-- In this collection, we will keep each title "as is" -->
<videos>
  <video>
	<title>The Distinguished Gentleman</title>
	<director>Jonathan Lynn</director>
	<length>112 Minutes</length>
	<format>DVD</format>
	<rating>R</rating>
  </video>
  <video>
	<title>Her Alibi</title>
	<director>Bruce Beresford</director>
	<length>94 Mins</length>
	<format>DVD</format>
	<rating>PG-13</rating>
  </video>
</videos>

This would produce:

Here is an example with various comments:

<?xml version="1.0" encoding="utf-8" ?> 
<!-- This is code for a real estate business.--> 
<Listing>
  <!-- A property is created with the Property element.
	For each property, a random number is created.
	This number is made of two parts that each
	ranges from 100 to 999.
	Both parts are separated by a dash.
  --> 
  <!-- The sale status, named Status, indicates whether
	the property is currently available or has already been sold.
	The possible values are Available and Sold.
  -->
  <Property Code="889456" Status="Available">
    	<!-- An element named DateListed specifies when the property was added 
	     to the listing or when it entered in the market.
    	--> 
    	<!-- As its name suggests, the YearBuilt attribute
	     indicates the year the property was built.
    	--> 
    <DateListed YearBuilt="2002">12/8/2008</DateListed> 
      	<!-- The property type must be
	 * Single Family: For a house that stands by itself
	 * Townhouse: For a full house but that is attached to at least another house.
	 * Condominium: For a type of apartment that belongs to a building.
      	--> 
    <PropertyType Style="Contemporary" Condition="Excellent">Townhouse</PropertyType> 
      	<!-- The style of a property indicates a type of architecture.
	     The possible styles are Contemporary, Colonial, Victorian, or Farmhouse.
      	--> 
      	<!-- The Condition attribute specifies whether the property can currently be 
	     presented to a customer, or at least how it appears at the moment.
	     The possible conditions are:
		* Excellent: This suggests that the property has little to no flaw
		* Good: This means that the property is mostly acceptable and can be 
		  presented to a potential buyer. In this case, something such as a 
		  refrigerator or a dishwasher may be missing; or something such as
		  the garbage dispenser may need to be fixed, or a piece of carpet 
		  that needs to be cleaned before the property is actually ready for sale
		* Needs Repair: This means that the property is in a good selling condition.
    	--> 
    	<!-- A property can be located by its address which
	     in most cases is made of an address, a city, a state (US, DE, AU, NG, etc) or
  	     province (CA, CM, etc), a ZIP Code (US) or postal code (CA, GB, BE, etc).
    	--> 
    	<!-- To make it easy to locally find a property, the location may not be an actual
	     city and usually people out of town may have a hard time finding it on a map. 
	     Nevertheless, local people, the inhabitants, and those interested in real 
	     estate properties would be familiar with the place.
    	-->
    <Location Address="1044 Larson Drive" City="Silver Spring" State="MD" ZIPCode="20904">White Oak</Location> 
      	<!-- Stories is the number of levels (in other countries such as AU).
	     This includes the basement (if any), the floor level, and the upper level(s), if any.
	     Most new constructions of single families in US, CA, DE, GB, and AU have three 
	     stories but you must check. Most condos have one story but it
	     is not uncommon for a condo to have 2 stories.
    	--> 
    <Stories>2</Stories> 
	<!-- A bedroom is a clearly designated area made for sleeping. It is easily 
	     identifiable in any construction. Still, if the basement of a house
	     is finished, unless it is clearly made as a bedroom, it should not be counted as such.
  	--> 
    <Bedrooms>4</Bedrooms> 
	<!-- In our real estate listing, there are two types of bathrooms, full and half:
		* A bathroom is counted as full if it is equipped with either a shower area, 
		  a bath tub, or both. In addition, it should have toilet and a sink
		* A bathroom is considered half if it (typically) has neither a shower area 
		  nor a bath tub. In most single family and townhouses, a half bath
		  is constructed on the floor level to conveniently serve the visitors who 
		  enter the house. A typical half bathroom has a toilet (to do the business) 
		  and a sink to wash the hands.
	-->
    <Bathrooms>2.50</Bathrooms> 
	<!-- The value of the property, also referred to as the market value or the listing 
	     value, can be the asking price for the property.
	-->
    <MarketValue>425860.00</MarketValue> 
	<!-- The description of a property gives an overview of what the property offers. It 
	     is usually written with positive words that can entice, we mean invite :), a 
	     pontential buyer to feel comfortable with the idea of acquiring the property.
	--> 
    <Description>N/A</Description> 
  </Property>
</Listing>

Since a commented section can contain anything, you can include HTML or XML markup in it. Here is an example:

<?xml version="1.0" encoding="utf-8"?>
<!-- In this collection, we will keep each title "as is" -->
<!-- Each video is created using the following formula
<video>
  <title>This is the name that appears on the package</title>
    <director>This is the name of the person(s) who directed the video</director>
    <length>This is provided as a natural number for the minutes</length>
    <format>The format can be VHS, DVD, BluRay, TV recording, etc</format>
    <rating>This is the MPAA rating</rating>
</video>
-->
<videos>
  <video>
	<title>The Distinguished Gentleman</title>
	<director>Jonathan Lynn</director>
	<length>112</length>
	<format>DVD</format>
	<rating>R</rating>
  </video>
  <video>
	<title>Her Alibi</title>
	<director>Bruce Beresford</director>
	<length>94</length>
	<format>DVD</format>
	<rating>PG-13</rating>
  </video>
</videos>