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.
Shows the class number of parts, reinforcement, and surfaces.
For assemblies and cast units, MAINPART.CLASS_ATTR shows the class number
of the main part.
For bolts, welds, and connections, CLASS_ATTR can be used for showing the
class number of bolted, welded, or connected parts. For example,
to show the class number of the bolt main part or the first
secondary part, use MAIN_PART.CLASS_ATTR or SECONDARY_1.CLASS_ATTR.