OK, here is the scenario:

I have this little snippet of a surface on a solid body that looks like it was created by a chamfer. This is an imported part and I just wanted to extend the surface so that it crosses my plane that I am working on.

So, I offset the surface 0" to effectively copy the surface.

Then I select "untrim surface" to 100% and select it.

However, this does not extend the surface as far as I want it to.

So, I thought about it....and I wondered if I could untrim it again.....

I would think that I couldn't...because I previously untrimmed it to 100%.....That is maximum...right?

Sure enough....selecting the surface that I JUST untrimmed and untrimming it again.

Now, it looks like a half truncated cone.

What if I keep selecting it?

Untrim # 4:

And, apparently, I can keep going, potentially extending the height of the cone infinitely.....

**So....what does "100%" really mean? How does the program know when to stop?**

I looked at the surface areas of the different surfaces and there really isn't a pattern.

Some of them are almost exactly 4 times the surface area of the previous one...some are almost exactly 6 times.

For some surfaces, it has a meaning. Like a sphere. 100% is easy to visualize for a sphere. A cone is bounded on one end, but on the other it could be infinite, as with other analytical types. But it doesn't really work that way.

You can use Extend where it works better, and sticks to the topological type.

100% has more meaning for a bounded general NURBS surface, where the math only goes so far. Extend can extrapolate, but the curvature gets unpredictable..

So from these experiments, for analytical types, 100% Untrim could/should be predictable, but isn't. For general spline surfaces, 100% untrim appears to be fully predictable.