'const' and 'in' parameter storage classes

[ref2401 via Digitalmars-d-learn]

What is the difference between 'const' and 'in' parameter storage 
classes?
When should I use 'const' or 'in'?

The documentation says 'in' is the same as 'const scope' but I 
can't write 'const scope ref' though it's legal to write 'in ref'.

Thank you