To explain picking, you can look at it like a sorting machine.
Imagine you have 30 different coloured boxes.
30 - Boxes
10 - Red boxes
10 - Blue boxes
10 - Green boxes
Pick all boxes - (Will pick 30 boxes)
Pick Red boxes - (Will pick 10)
and so on.
Imagine that each of these boxes have a variable that hold a number from 1 to 3, which are random for all the boxes no matter what color they have.
If you have an event like this.
Pick all boxes who have a value of 1 (Lets assume that it pick 8 boxes)
--- (Sub event) Boxes need to be red (This will pick all red boxes of those 8, so it might pick 2 boxes.)
--- (Sub event) Pick all Boxes (Will pick all 30 boxes because it is a system command)