Assigning parameter on entry to a function and assigning back on exit

[Victor Porton via Digitalmars-d-learn]

Somewhere in my brain memory, it was written:

A function argument that is both input and output, may be passed 
to the function either as reference or do two assignments: on 
entry of the function it is assigned to the parameter, on exit it 
is assigned back. Whether it is a reference or two assignments 
depends on the reference semantics of the type.

Now I can't find this in the reference manual. Please help to 
refresh/correct my memory.