Dimitre Novachev correctly points out on xsl-list:

In section "6.6 Modes" of the 2nd Last Call of the W3C XSLT 3.0
specification (http://www.w3.org/TR/2014/WD-xslt-30-20141002/#modes)
we read:

"[Definition:  Modes allow a node in a source tree to be processed
multiple times, each time producing a different result. They also
allow different sets of template rules to be active when processing
different trees, for example when processing documents loaded using
the document function (see 20.1 fn:document) or when processing
temporary trees".

This is true, but the definition mentions only processing "a node" and
leaves out the fact that a mode may contain templates that match an
atomic item or a function.

In the whole text throughout this section, only processing of nodes is
mentioned and this leaves the perception that it isn't allowed for a
mode to contain templates that match non-node items.

....
-- 
Cheers,
Dimitre Novatchev