I have been reading a Statistics book and they show the following formula for z-score:

$z=\frac{x-\mu}{\sigma}$

And then they explain about the $\mu $ and the $\sigma $ in the formula like this:

These are the mean and standard deviation of data containing the value x.

But is that so ? I don't think the mean and standard deviation should contain the value x.

$z=\frac{x-\mu}{\sigma}$

And then they explain about the $\mu $ and the $\sigma $ in the formula like this:

These are the mean and standard deviation of data containing the value x.

But is that so ? I don't think the mean and standard deviation should contain the value x.