“Subject to” is one of those bits of technical language that can easily cause misunderstandings for the unwary. It comes up frequently in legislation and often leads to mistakes.
A rule that is described as subject to something else, is subordinate to that thing. So, if Rule A is subject to Rule B, Rule A is subordinate to Rule B.
In other words, it will take effect, only so far as Rule B does not apply.
This is much easier to see in practice.