<stock>
<add>
<item>
<name>Milk</name>
<quantity>2</quantity>
</item>
</add>
</stock>
Putting this body to an resource like /stocks/123 violates the idea behind REST.
While this body is put and it contains all informations neccessary, it also comes along with an method call to add somewhere when the body is processed.
Following REST one would post the item to /stocks/123/items/.