Data Services sizing dashboard - how is delta load calculated

I used below dashboard for sizing of on-going transaction load

http://www.sdn.sap.com/irj/scn/go/portal/prtroot/docs/library/uuid/00da5fab-4908-2d10-e3b7-c59b9719f76d?QuickLink=index&overridelayout=true

I am confused what is initial load, delta load mean for a daily load. They have below formula used for load time

Load_time = overhead_time + number_of_rows_overall / rows_per_second

Load_time = overhead_time + (number_of_objects * average_number_of_rows) / rows_per_second

but i am not clear how initial and delta load is differential from my input of number of rows, number of dimensions.