Is there anyway that user can decide that "at this point, control point should appear" (i mean internal ones, not the external 4 point that are on extreme ends and are paired)
Or what are the factor that decides internal control points?
If I understand the question right, you could use multiple connected curves to provide more "ends".
I have a hard time with complex boundary blends but I have had luck connecting multiple sections that have more than 1 curve in them. Maybe this is what will give you the extra control point you are looking for.
There are also other methods such as sweeps-blends, and even some legacy features that do something similar including adding influence curves.
Here it is with natural it gets skewed and with arclength it gets all arranged,but don't know how to pair control points i mean is it just arbitrary,like opposite opposite.
(also is everyone getting moderated again?)
This one simply begs for a variable section sweep...
I don't know why the boundary blend is failing. It almost acts as if something is crossed.
Anyway, this is a sweep with one contour as the origin and the second as a "guide" (chain1).
I don't think the control points are arbitrary, your two curves have different numbers of entities – 6 in one, 8 in the other, and also these bits of curve are of different lengths to one another so internal control point pairs get created as this information is sent from one curve to another.
It might not be what you want... ... ... but to look at the boundaries, if you create a composite (copy) curve of the boundaries and use approximate (to get two curvature continuous boundary curves) and then create the blend you will see that it makes an OK surface the major issue being the tight corners, there you will need to create some control points I think.
To add control points you do not have to match up all pairs all the way around, you can just add in some helping pairs. In the example where you create the composite curve there will only be one internal point (big green cross) per boundary.
Control point pairs can go from internal (big green x) points to datum points created by you; or from internal point to internal point; or from datum point to datum point. (The datum points need to be of the type 'on curve' I think)
You can also add datum points onto the control curve and then join internal points to datum points.
You can explicitly tie pairs of points together in the control points dialog. The UI isn't very friendly, but it is possible.
Open the control points dialog and then click in the top row of the lower right pane. The points of your first curve will highlight in green (in Creo2).
Choose a point on that curve and then the points on the second curve will highlight and you can choose the corresponding point. You go through picking pairs until you've paired all you want to. You don't need to pair them all, but I found that doing all 6 in the one curve to 6 of the 8 in the other produced good results.