What that says is that the boot files must be in a Primary partition
that is marked "active". It does NOT say that the boot files are
put into the partition that happens to be "active" at the time that
the linstaller starts installing the last OS. In fact, the very next line
in your quoted document says
"The system partition can, but is not required, to be the same
partition as the boot partition."
In other words, it is completely arbitrary which partitions contain
the boot files and which contain the OS. That last partition that is
used for an installation of an OS, i.e. partition E: that is used for the
installation of WinXP, could just as well be used by the installer for
the boot files and could just as well be marked "active" by the installer.
Marking a partition "active" is very easy to do - some cloning utilities
automatically mark as "active" the copied partition that they produce
whether or not the partition which they copy was originally marked
"active".
What you say may be true - that the installer puts the boot files in
the pre-existing "active" partition - but nothing as far as I can see
requires it to do so because it can put the boot files more easily in
the partition where it is doing the installation, and to mark that partition
as being "active" is a trivial thing for it to do. So why must the installer
for the WinXP OS, i.e. the last in the series of OS installations, put its
boot files into the partition which contains the 1st OS to be installed,
i.e. the partition which contains Win98?
*TimDaniels*