PointIn1 is the name of the first of the position controllers in the CustomTool. If you mouse over any input in the Inspector, you'll see its name appear in the lower left corner.
Point controllers, of course, have two dimensions, so if you need to address them separately, there's a way to do it. The Point() function converts two numbers into a single Point vector. Since you can't put separate expressions in X and Y, you have to use Point( <x expression> , <y expression> ) to do it.
To read numbers
from a Point, you can take them right out of the vector components .X and .Y.
Thus, if I want the X portion of Transform1's Center control, I'll mouseover the control to find out that it's name is, indeed, Transform1.Center. Then I can grab that X value with Transform1.Center.X.
The CustomTool is, by far, my favorite Fusion tool, and it's rare that you'll see a comp come from my desk that doesn't have one in it. Obviously, the availability of all of those Points and Sliders is great, but it also lets you do all kinds of interesting math on an image, using up to two other images as controls, on a pixel-by-pixel basis.
I've written about it here:
http://www.bryanray.name/wordpress/blac ... pressions/Along with a general tutorial on procedural animation using expressions.
For your tracking problem, I have not encountered that particular crash scenario before. You might try setting Use GPU to Disabled on the Tracker. That's usually the first place I start to troubleshoot mysterious Fusion problems.