XQuery Update - unordered collection of update primitives

Section 2.1, Extensions to the Processing Model, carries the following
definition.

[Definition: A pending update list is an unordered collection of update primitives, which represent node state changes that have not
yet been applied.]

What is the significance of an <<unordered>> collection of update primitives ?
How is updating integrity preserved with an unordered collection of update
instructions ? I could not find an explicit description on the above and
would appreciate clarifications.

Thank you.

Kuan Hui