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 top level of a single part, cast unit, assembly, part of a connection, or a pour object.
Top level takes the unit and accuracy from MarkDimensionFormat.dim dimension property file. You can modify the settings saved in MarkDimensionFormat.dim in the dimension property pane in an open drawing.

The datum level only affects the
TOP_LEVEL attribute when
Location by is set to
Model origin or to the project base point that is in the model origin.

You can use this attribute as a user-defined attribute also in part marks and associative notes.
This attribute returns the value as text, so you cannot use formulae with this attribute. Use TOP_LEVEL_UNFORMATTED instead.