The selec flag is used to define the axes involved in the regulation of the constraint.
For example, the `height` constraint is a distance only on the Z coordinate, while the old `distance`  constraint corresponds to a the regulation of norm on the X,Y coordinates.
For a constraint that does not regulate the norm of the error, but each coordinate in a separate way, the selec indicates the axis considered, and the number of axis considered should correspond to the size of pos_lo and pos_hi.

Example: I want the hand to be above a table: I will constrain the X and Y axes only.
<pre>
pos_lo = [-0.1, -0.2]
pos_hi = [ 0.1,  0.2]
selec   = '011' # constraint the X,Y axes
</pre>

For now, the selec element directly corresponds to the selec that can be found in the sot Feature class, aka a table of ints '1'/'0' indicating which dofs are involved or not, which is not intuitive.
I used that in my tests, but this has to be changed in the future.