More detail needed for Sheet and Sheets functions

How should these functions behave around hidden and very hidden sheets? What about 3D-refs to SHEET?
What order we are talking about? Order in sheet bar? Any internal order which guarantees unique ids?

What happens with cases where formula evaluation is handed off across sheets?
For example,
In sheet1: =Sheet2!foo, where Sheet2!foo is defined as =SHEET(!B2). The top-level formula is on Sheet1, but foo is on Sheet2, but !B2 evaluates to a cell on Sheet1.
Or what if you're calling a macro function on a cell in a normal worksheet and the macro function calls SHEET – which sheet do we return?