Udev rules files are sorted and processed in lexical order.

When looking at standard rules, we see:

# ls /lib/udev/rules.d
50-firmware.rules
50-udev-default.rules
60-block.rules
60-cdrom_id.rules
60-drm.rules
...

90-vconsole.rules
97-hid2hci.rules
99-systemd.rules

But in /etc/udev/rules.d, there are some files without any prefix:

# ls /etc/udev/rules.d
72-pvr-seat.rules 
automount.rules
lightmediascanner.rules
touchscreen.rules
udev-smack-default.rules

This leads to unexpected behaviour regarding rules priorities and precedence. The files should have a 2 character prefix to specify priority.