ARM Cortex M4 PLL

Hi, i understand the concept of PLL however i am struggling to understand it in the context of the arm cortex micro. Specifically the relationship between bus speed and clock speed.

An extract from a book I'm reading states:

Assume we wish to run an LM4F/TM4C micro with a 16MHz crystal at 80MHz. The divide by 2.5 creates a bus frequency of 80MHz, implemented as 400MHz divided by 5.

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from my understanding:

the "SYSCTL_XTAL_" instruction is setting the speed of the crystal so what is the relevance of the 80MHz. I know that it is the bus frequency and if that is the care then by which frequency does one judge the speed of the microcontroller.