You can mark or check certain work center to be bottlenecks. This means work centers that should be taken into special consideration during the loading plan.
A bottleneck is a selection term used during loading plan printout, in order to only see the work centers that are of special interest with regard to loading. If your manufacturing does not have any work centers that should be taken into special consideration during loading, or if the bottlenecks in your manufacturing are very flexible/variable, then you do not need to use this field.