Where does the OS's for the dropdown for Price Model->Other Cost come from

In Chargeback, under the  "Manage Cost" in the "Price Models" under the "Other Cost" you can set the fix cost for guest OS.

The "Guest OS Name" dropdown at different times has new OS's in it. I attached a pic to show what I mean.

Where are these OS's coming from?

When is the list updated?

Thanks in advance,

Keith