Trigger an operator on Select/Deselect events

Hi dear coder friends,
I want to make an operator that is triggered when the selection change in edit mode (select/deselect) or at least when changing active vertex.
Is it possible to do it with a handler ? I already tested handlers listed by CoDEmanX on this page (pretty old) but nothing seems relate to selection.

I think the trigger may just be a modal with detection of mouse click event (even for this, I don’t really know how to proceed). But I think I’ll do it as a last resort, it’s certainly more “economic” to make it react to change in selection state as it’s supposed to act on selection, and it might be triggered involontarily with mouse click (when using Gpencil or something else…)

any suggestion ?

To be more specific on what I want to achieve.
When selecting a vertex in edit mode I want every other aligned vertices (in view Z) to be selected as well (only when the option is On, in edit mode and in ortho view).

My code is already working (exept for detecting ortho view). I’m just missing the trigger part.