This was taken from Pro/E help:
Importing a Datum Curve
An imported datum curve can consist of one or more segments. Multiple segments are not necessarily connected.
The From File option imports a datum curve from a Pro/ENGINEER .ibl, IGES, SET, or VDA file. Pro/ENGINEER does not automatically combine the curves imported using From File into a composite curve.
Pro/ENGINEER reads all the curves from an IGES or SET file, then converts them to spline curves. When you import a VDA
file, the system reads the VDA spline entities only.
The .ibl file format is very much like that of a blend file, except you should precede the coordinates of each segment of the curve with both begin section and begin curve. Two points in a section define a line, while more than two define a spline.
To connect curve segments, make sure the coordinates of the first point are the same as the last point in the previous
section.
You can redefine datum curves that are created from a file and you can trim or split them with other curves that are imported from a file.
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Example: Sample Datum Curve File
A simple example using arclength blending is given in the help.