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 prefix used for a layer of rebar set bars.
You can define the default layer prefixes of a model in the Options dialog. You can also define bar layer prefixes for individual rebar sets using their user-defined attributes, or for individual rebar set bars using the user-defined attributes of property modifiers.