XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (440 page)

The syntax also allows several sequences to be provided as input, in which case the effect is to apply an expression to every combination of values taken one from each sequence.

Expression	Syntax
ForExpr	for $ VarName in ExprSingle ( , $ VarName in ExprSingle)* return ExprSingle
VarName	QName

An
ExprSingle
is any XPath expression that does not contain a top-level
,
operator. If you want to use an expression containing a
,
operator, write it in parentheses. For example, the expression
for $i in (1,5,10) return $i + 1
returns the sequence
2,6,11
.