As Chris_TC and gray pointed out, a point can be associated with multiple polygons. So you use VertexToPolygons to either build a per-point array of polygon attribute values, or you do something like average the polygon attribute values and store that as a point attribute value.
ICE attributes are categorized as built-in or user-defined, so you would assume you could easily find your own custom attributes by checking the AttributeCategory property. However, try this: create an empty point cloud and run this script:
si = Application attrs = si.Selection(0).ActivePrimitive.Geometry.ICEAttributes; # There's also a "unknown" category but I'll ignore that here for attr in attrs: si.LogMessage( "Attribute: %s, IsDefined: %s, AttributeCategory: %s" % (attr.Name, attr.IsDefined, "Built-in" if attr.AttributeCategory == 1 else "User-defined" ) )
You’ll see that while some attributes, like NbPoints, PointPostion, and PointVelocity are “built-in” as you would expect, most of the attributes are in the “user-defined” category.
For example, size and shape are listed as user-defined attributes. Why’s that?
I think it is because those attributes are dynamic attributes added by [factory-default] ICE compounds. Until you plug in those compounds, the compounds don’t exist and aren’t initialized (in other words, they are not defined yet). They’re not really built into the system (and the attribute explorer is hard-coded to show them as a convenience).
Built-in attributes like PointPosition are intrinsic attributes.
If you want to distinguish your own attributes, I’d use a prefix for the attribute names. For example, I sometimes use a “ps” (for Product Support) prefix or a “sisupp” prefix.