How is the Rate Base Item "Source Wait Time (duration spent waiting at stop)" calculated?

Summary:

I wonder how the Rate Base Item "Source Wait Time (duration spent waiting at stop)" and as well the same for "Destination" is calculated.

Content (required):

I wonder how the Rate Base Item "Source Wait Time (duration spent waiting at stop)" and as well the same for "Destination" is calculated. I assume that this is based on events entered in the system against the according shipment but as there is no description for this RBIs in the help I have no idea about the details, so which events are driving the process of the calculation for this value?

Version (include the version you are using, if applicable):