XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (205 page)

and
that are classified as alphanumeric).

The punctuation token that precedes the first formatting token, if any, is called the prefix. The one that follows the last formatting token, if any, is called the suffix. The other punctuation tokens are referred to as separators.

In the most common case the place marker is a single number. In this situation, the output string consists of the prefix, followed by the result of formatting the number using the first formatting token, and then the suffix. So if the place marker is
42
and the format attribute is
[1]
, then the final output is
[42]
.

Where the place marker is a list of numbers, the rules are a little more complex but still have intuitive results; for example, if the list of numbers is
3
,
1
,
6
and the format attribute is
1.1(a)
, then the final output is
3.1(f)
(because
f
is the sixth letter in the alphabet). The detailed rules are as follows: