Questions about 12.1.0.2 sys.heatmap

Hi, DBAs

This is 12.1.0.2 on Oracle Linux 6.  The database is 3.5 TB, mostly read-only content loaded over a few days.  SYSAUX contains a 500 MB segment named SYS.HEATMAP, segment type system statistics.  I've not noticed this before; here, it's the largest segment in that tablespace.

Parameter heat_map=off in the database.  Other 12.1.0.2 databases also have a small segment named sys.heatmap, but these are less than 10 MB each.  A few questions:

How is sys.heatmap maintained?

Is sys.heatmap related to parameter heat_map?

How do I estimate how much should be in sys.heatmap?

The information I've seen on heatmap for the RDBMS is for the IM feature.  I'm wondering what else relies on sys.heatmap.