How is Threshold parameter calculated and expressed in Unsharp Mask?

Hi,

I'm trying to understand the imsharpen function:

https://it.mathworks.com/help/images/ref/imsharpen.html

I would like to understand how the Threshold parameter is calculated. It is about the minimum contrast, but how the contrast is calculated? The Threshold must be a scalar in the range [0 1], so I think it is a ratio: but of what?

As example, if I want to exclude the edges characterized by a difference of intensity equal or less than 20, which is the value to set as Threshold?

I also tried to investigate by using the imgradient function, but the resultant matrix can have values greater than 1, so I think it is not what I'm looking for.

Finally, in my case it could be very very useful to return a matrix (of the same size of the image) containing the value of contrast pixel by pixel, as calculated in order to apply the Threshold.

Thanks for your answers, kind regards.