Which are the elements of the Twofold Screw Axis in this image?
The "+" and "-" elements that intercept the red line are the elements of the Twofold Screw Axis 2/3?
And if so, is the height of the red line 2/3 plus or 2/3 minus?
But why the height of the red line is 2/3?
I have the same problem with the other colored lines. Are the elements of the green line 1/3 minus or plus?
I would strongly recommend to consult the references I mentioned in this answer earlier: R32 Space Group (Crystallography)
In particular have a look at the meaning of the individual symbols and how space groups are represented in these kinds of diagrams. Again, the International Tables of Crystallography, Volume A, has plenty of explanation on this in its first part. I would also recommend not to start out with the rhombohedral groups, as they are probably the trickiest of them all due to the slightly uncommon choice of centring. Try to start with an orthorhombic space group, for instance, at least until you do not have troubles with the symbols any longer.
As for an answer: your question unfortunately does not make a lot of sense to me. I'll go with a hunch here: Could it be that you are mistaking the "+" and "-" signs for something they are not, like having anything to do with height (which they do not)? They describe the orientation of the motif - think "upwards-/downwardsfacing". Hence (screw-)axes obviously do not have a "sign", but change the sign of the motif they operate on.
