XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (425 page)

To see the difference, consider the following source document:

With this document, if the variable
$chapters
contains only the outer

element,
$chapters//diagram[1]
will select diagrams 12, 14, and 15, while both
$chapters/descendant::diagram[1]
and
($chapters//diagram)[1]
will select diagram 12 only.

Combining Sets of Nodes

Until now this chapter has been all about path expressions. This section describes operators that combine two sets of nodes. Although these aren't technically path expressions, they are invariably used in conjunction with path expressions, so it's useful to deal with them at the same time. The operators are:

• Union, written
  union
  or
  |
  , which includes a node in the result if it is present in either of the two supplied sets, eliminating any duplicates.
  
• Intersection, written
  intersect
  , which includes a node in the result if it is in both the two sets.
  
• Difference, written
  except
  , which includes a node in the result if it is in the first set and is not in the second.