XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (442 page)

where
F($x)
represents any expression that depends on
$x
(it doesn't have to depend on
$x
, but it usually will).

What this does is to evaluate the expression
F($x)
once for each item in the input sequence
$SEQ
and then to concatenate the results, respecting the original order of the items in
$SEQ
.

In the simplest case, the return expression
F($x)
returns one item each time it is called. This is illustrated in
Figure 10-1
, where the function
F($x)
in this example is actually the expression
string-length($x)
.

Figure 10-1

We say that the expression
for $x in $SEQ return string-length($x)
maps
the sequence
“red”,“blue”,“green”
to the sequence
3,4,5
.