Jesus,

I've a box and just want to scatter the top polygon of it. I select it, it turns to yellow but as soon as I connect the scatter SOP the whole box is scattered. So is it me?
Thanks in advance

You should be sure that the Mouse cursor is on the viewport (Not Network View).

If every things works good ,when you append Scatter SOP ,you should see primitive number in the Group field of the Scatter SOP 8)