thanks for the answer but I'm afraid you've misunderstood my question:
Mind you, I'm not asking whether it should be wise or not to apply more than 100% USM in a picture - I'm wondering why the Unsharp Mask filter has been engineered by Adobe developers with such a ceiling value.
I mean, the 500 ceiling is arbitrary? If the unit of measure is in percentage, why is that the maximum allowed value is not 100%?
My question in this case is from a developer point of view if you will, not from a retoucher one.
For the Radius parameter, this is clearly expressed in pixels as the radius of the Gaussian blur used to compute the difference (Unsharp Mask needs a blurring kernel, it works that way). It's just not clear to me why the Amount range goes up to 500%
Thanks anyway Erik!