Tunable vs. internal properties

[Patrick McCarty]

Hello,

How are tunable and internal properties distinguished in the source
code?  I am speaking of both context properties and grob properties.

I can see that they are hardcoded in scm/define-context-properties.scm
and scm/define-grob-properties.scm, but I am having trouble
understanding how some properties are "tunable" but others are not.

Can anyone point me to specific functions I should be looking for (in
either C++ or Scheme)?

Thanks,
Patrick