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.
Drawings
The IS_FROZEN attribute tells if the drawing is frozen. This attribute can be used for adding Document manager information about frozen drawings in drawing reports. The report returns the value 1 if the drawing is frozen, and 0 if it is not frozen.
Document manager has a column 
Freeze for this information.
In the image below, you can see that some of the drawings are frozen (a flag in the Freeze column).
Rebar sets
With the REBAR and SINGLE_REBAR content types, the IS_FROZEN attribute tells if a rebar set is frozen.
The attribute shows 0 if the rebar set is not frozen, 1 if the rebar set is frozen, and 2 if the rebar set is partially frozen, meaning that some of its leg faces are frozen and some are not.