Offset is usually only 2 - 4 degrees. When you get much past this you start getting into helical.

There really is no differance in the two because the fletch will wrap around the shaft when it is fletched other than perfectly straight.

The reason why they call it straight offset is because the vane or feather is not wraping enough around the shaft to use a helical clamp. A helical clamp has a wrap built into it and allows the fletch to wrap around the shaft and get 100% contact. A straight clamp would only make contact with the middle of the vane when you get past a few degrees.