All function is used when you want to fetch data from the entire table or a few columns without filter action.
All except is used similarly to all but would fetch data that is affected after applying filters from the entire table or few columns.
Both have the same function syntax -
syn = ALLEXCEPT/ALL(table name,column name).
Both would fetch multiple values this need to be used along with calculate() or sumx(), etc functions
As per your question,
create a column as count(orders
Create a table with a count of orders and region.
Now create a column to find count(orders)/distinctcount(cust-id) to find the orders per customer.
Now drop the field beside region to get the average order per customer for every region.