You don't "have" to order the prepare PTF now. It will apply itself when
the TR comes out by automatically doing a series of IPLs. But you may
want to read the FAQ at:
Note: A new TR will always mean a new resave of the OS will be available.
This is documented at the above TR site.
And this site says that a TR normally means that these cover LIC code and
just the OS that the LIC needs fixed. But often the cool stuff one is
looking for is released by a group ptf that might have been waiting on
dependancies of this new TR. For example, new function may come out as a
new DB2 group that comes out after a TR. While the new function may be
dependent on the TR, it comes out differently. Technically this means
that if you order SF99717 for the latest TR for 7.2 you may also want to
order the other groups, like SF99721 for database.