>When do I need a private or a defalt datasession. Wat are the advatages end the disadvatages when we only open the screens once on a computer but we need acces for more computers.

Private DS has nothing to do with multi-user problems. Essnetially, it's the way to 'separate' data between different objects (forms) within application running on the same machine. If you use multiple modeless forms with buffering and bound controls then private DS is 99% required to use.