XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (415 page)

The meaning of this expression is: the node that is the root of the tree containing the context node, provided that this is a document node.

The symbol
/
is unusual because it is used both as an operator and as an expression in its own right. This can lead to some syntactic ambiguities; for example, the expression
/ union /*
looks as if it is trying to find the union of the two node sequences
/
and
/*
, but actually it is an absolute path expression whose first step is
child::union
(
union
is a legitimate element name) and whose second step is
child::*
. If
/