Read XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition Online
Authors: Michael Kay
IdKeyPattern | Matches a node with a given ID attribute or key value. For example, id(‘A001’) matches an element with an ID attribute whose value is A001 . |
IdKeyPattern / RelativePathPattern | Matches a pattern defined relative to the children of a node with a given ID attribute or key value. For example, id(‘A001’)/title matches the A001 . |
IdKeyPattern // RelativePathPattern | Matches a pattern defined relative to the descendants of a node with a given ID attribute or key value. For example, id(‘A001’)//title matches any A001 . |