Sometimes I see a glimpse of a crossbar on top of the rollcage ( runs from corner to corner of the main hoop and halo bar ). Is it always one bar or are they sometimes crossed ( X shape ). What is the main purpose of this and how are they usually installed, anyone got some pictures of this?

I dont have a pic handy but it keeps the halo from
racking, normally its from the left front to right rear

If it was to be crossed [ x brace ] it would probably interfere with the head room as it would have to be padded also.

What Mr. P body said.

Figure the chassis wants to twist the left front corner up, that force transfers through the halo cross bar and pushes back and down on the right side of the main hoop and the rear down bars. very effective triangulation. With that said I didn't install that one in mine.

Now it makes sense.