Couverture is French for "Covering", so it is more fluid than regular chocolate. This is achieved by adding more cocoa butter to the chocolate. Generally for dark couvertures 55% cocoa content is the starting point.
I guess we should go all the way back to the cocoa bean. After roasting and crushing, the resulting substance is referred to as "Cocoa mass" or "Cocoa liquor" it is 100% pure cocoa bean, and as such, it naturally contains over 52% cocoa butter. Even so, when warm it has the consistency of peanut butter. So, for dark chocolates, you will find cocoa mass listed first on the ingredient list, followed by sugar, then cocoa butter, then vanilla and then soy lecethin. Each of the last two ingredients are less than 1/2 of one percent. Obviously, the more sugar in there, the less cocoa content is in there. So for a 55% dark couverture, you have about 44% sugar.
Milk couvertures have the addition of milk powder. Generally about 1/3 cocoa mass, 1/3 milk powder, and 1/3 sugar. Usually you'll find good quality milk couvertures with about 35% cocoa content at the starting point.
White couvertures only have cocoa butter, milk powder, and sugar. The cocoa butter is the cocoa content even though it has no flavour.
So you will find that the 29% white will be less fluid then the 35%. For molding shells and bon-bons, this does make a difference, as your shell will be thicker, but for dipping or ganaches, the 29% is pretty good.
May I suggest a book? It is called "Chocolates and confections" by Peter Grewling, an instructor at the CIA (Culinary inst.america) this book will provide an enormous amount of information. Your library should have it, and I have seen it at many bookstores, even the chains like Chapters and Indigo. Although it is pricey (aprox $80) no one says you can't have a good peek at it in the bookstore, but try your library first.
Hope this helps