Hello Vladislav,

super(sale_order, self)._prepare_order_line_procurement(
            cr, uid, order, line, move_id, date_planned, context=context)

This statement is called special kind of overding in Inheritence concept.

 

Lets take a  simple example -

class Parent():
    def my_function(self,a,b):
        print 'Hi iam parent class'
       

class Child(Parent):
    def my_function(self,a,b):
        print 'Hi iam child class version 1'
        super(Child,self).my_function(a,b)
        print 'Hi iam child class version 2'

##create instance

obj=Child()
obj.my_function(1,2)

#output

'Hi iam child class version 1'
'Hi iam parent class'
'Hi iam child class version 2'

In your case you can find parent class function _prepare_order_line_procurement  in sale_stock module at line:292

Thanks