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 template attribute shows the image ID in the format <Id>-<SubId>. This attribute is used in reports and in some exports when exporting image ID information, for example, in the BVBS export.