AI Focus & AI Servo - MKIII, what AF point selection? Back button focus

Hi there,

I have a 5D MKIII. I have started using back button focus.

I have removed focus from the shutter button.

If I press the AE-ON button it will focus.

This has resulted in me trying to understand tracking and focusing points.

Many video's have been watched. Manual has be read.....still I am a bit confused

If I have the camera set for either AI Focus or AI Servo & I focus on a subject which then starts to move it seems to track it but only in the centre circle. It doesn't track outside the centre circle

I have the centre manual selection chosen as my focus point.

I thought it would track a subject across the other focus points Maybe I am missing something?

If I have selected the centre focus point will it only only use this focus point?

This I guess would mean if I had a subject running at me straight down the centre of my camera then it would keep focus on the centre point? Maybe that the tracking it does?

Any help advice would be appreciated

Thanks