Work Breakdown Structure (WBS) is a way of breaking your projects down into smaller chunks or pieces. 

Legacy versions of aSa allowed you to specify segments, sub-segments, bid items, bid groups, and other identifiers for organizational purposes. aSa.Studio is more flexible. It allows you to create as many levels as you want up to 9 and call them anything you want. For example, you may call your levels segments and sub-segments, or name them something different altogether, such as zone or area. Basically, they are all just levels within the structure of the project. 

WBS is defined for each project in Job > Tools > Job WBS Template. Refer to Job documentation for additional details.