Hi Bas - the ‘domain’ of a curve or surface, that is the range or size of it’s ‘parameter space’ can be any two (increasing) numbers - Re[arameterize lets you set those numbers and the Automatic option sets the numbers to something close to
0>3d size of the object.
For example, draw a new control point curve with a few points - measure its length and also its domain (
Domain command). Then move points around to make the curve noticeably longer or shorter - the length will not longer match the parameter space domain of the curve which remains as it was when you created the curve…
Reparameterize > Automatic will reset that to match the length. Similar for surfaces though there of course its an approximation unless the thing is a simple rectangular plane since size is not constant.