If the basis curve C is periodic there is an ambiguity because two parts are available. In this case by default the trimmed curve has the same orientation as the basis curve (Sense = True). If Sense = False then the orientation of the trimmed curve is opposite to the orientation of the basis curve C.
2D Curve to trim
First param on the curve for trimming. U1 can be greater or lower than U2. The returned curve is oriented from U1 to U2.
Second parameter on the curve for trimming
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If the curve is closed but not periodic it is not possible to keep the part of the curve including the junction point (except if the junction point is at the beginning or at the end of the trimmed curve) because you could lose the fundamental characteristics of the basis curve which are used for example to compute the derivatives of the trimmed curve. So for a closed curve the rules are the same as for a open curve.
true