ROI Types

Point3D

It is actually not a point, but rather a sphere around a single point in 3D. It is rendered as a circle (in Outline/Wireframe modes) or a disk (in Surface render mode) that is positioned in a plane perpendicular to a viewing direction.

Polyline3D

Represents a set of points connected by straight lines (or by interpolating spline).
This ROI has a "direction", i.e. points in it (and so measurements) are ordered
from one end (beginning) to another (end).
The last point (end) is rendered with an extra square around it.

LineTrace3D

Represents a set of reference points connected by curved segments that follow maximum "tubeness".
This type of ROI can be created using Semi-auto-tracing or One-click-tracing modes.
This ROI alse has a "direction", i.e. points in it (and so measurements) are ordered
from one end (beginning) to another (end).
The last point (end) is rendered with an extra square around it.

CrossSection3D

Represents a plane fitted to a set of points. It is limited to the part of the plane crossing the dataset.
Since a plane is defined by a minimum of three points, the cross-section plane will not be shown until at least three points are added.
If more than three points are added, the position of the plane is defined by singular value decomposition, see this link for details.