XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (267 page)

The name of the parameter is defined by a lexical QName. Normally, this will be a simple name such as
city
or
total-sales
, but it may be a name qualified with a prefix, for example
my:value
. If it has a prefix, the prefix must correspond to a namespace that is in scope at that point in the stylesheet. The true name of the parameter is determined not by the prefix but by the namespace URI; so the name
my:value
will match a parameter declared as
your:value
if the prefixes
my
and
your