You can apply a line snapper (found on Capture's Built-in library) on the vertical pipe of the stand and type the word Fixture on the Match field of the snapper.
Do not forget that you can use the break group tool on the stand and align the vertical pipes to the desired height.

Hi, it's ok for the Line Snapper.

But, i want the fixture to be hanged upside. (PAR64 for exemple)

Using Manfrotto Spigot in this case...

Can't achieve it with rotations...

I'ld like to be able to rotate around the Clamp...

Hope this would be clear :-)

Thanks in advance

To snap a fixture on top of the tube or a stand in your case, we recommend using a pair of point snappers from our built-in library in order to make the objects that you wish to snap to each other. A pair of point snappers snap to each other like magnets, without altering the orientation of the snapping object. Additional information is available in the relevant section in our manual.

I managed with some difficulty to hang a Fixture above a manfrotto stand. 
This requires additional work.

Here are the steps taken :

1. add a stand
2. add a Point Snapper, and placed it on the top of the Stand, as would be the “Spigot”
3. set the Match property of the Point Snapper  to “Truss”.
4. add a second Point Snapper, with Match property set  to “Truss” too.
5. add a Fixture (PAR for exemple)
6. place the #2 Point Snapper at the location of the Fixture Clamp
7. Then, i grouped the #2 Point Snapper and the Fixture (Point Snapper disappears)
8. Need to rotate the Group Upside, to keep the Clamp at the bottom
9. Then Moving the Group and snap it to the Manfrotto

Is this the right way to achieve that ???

This seems to me to be rather complicated to implement, for a very common need.

Why aren’t the stands ready to receive snapping fixtures ?

A demonstration video would be very helpful

Thanks