This is a template attribute. Template attributes can be used in filtering, or to retrieve data from the model to use in drawings and reports. For example, when you create a report, Tekla Structures replaces the attributes added in the report template with the corresponding object property values.
This attribute shows the volume of a solid weld object. If the solid weld object fails, the attribute shows 0.00. The attribute shows 0.00 also for unsupported weld types.
You can use this template attribute also to calculate weld weight, see the article How to show weld weight on templates and reports for more information.