I'll try
The session itself is quite structured, IME this was circle time, 'work', snack and then group play.
When the children are 'working' the way in which they work is quite structured, i.e. they must take the activity from the shelf, get a green mat (to define their work space) put the activity on the green mat, work with it until the activity is completed then make it clean and tidy for the next child, then put it back on the shelf, then get a new activity from teh shelf and so on. If a child wants to join in with an activity someone else is already doing they must ask permission to join in, the child using the toy can let them or not, it is their choice!
However, the child has pretty much free rein on what activity they choose to work on. So they can make the choice from the various activities set out on teh shelves. Some things like trainsets, dressing up box, climbing frame, can only be used in the group play session, but any other activity can be chosen. In reality the child does get an element of direction from the teacher in helping them to choose what to do, so the teacher will keep an eye out to make sure that the child gets a range of experiences over the course of the week. Although when they are smaller this happens less than when they are older.
So I suppose the order and structure comes from the way in which they work, the choice comes from what they choose to work on!
If it helps, the way in which they work is so calm and focussed they are used to sitting down and concentrating on a series of tasks for 1-1.5 hours at a time. This serves as excellent experience for the classroom situation where some children (boys mainly) just cannot seem to sit down for more than 5 minutes at a time!