Hi
Source structure and allocation structure is used as default object type to which the internal order will propose to settle if more than one is available.
Source structure:
You can use the source structure to create distribution rules for different groups of costs.For example you may want to settle personnel costs to a specific cost center.
Allocation Structure:
in allocation structures we create group of accounts that will be settling to a secondary cost element (Assessment). For example all of the labor-related costs from a cost center will be settled using a specific secondary cost element.
hope the above information makes it clear for you..
Regards
Mahmoud El Nady