The "ac" prefix indicates an Access intrinsic constant, where as the "Ac" prefix indicates an Access intrinsic enumeration (of constants).
An Access intrinsic constant is simply a predeclared, symbolic constant defined in the Microsoft Access x.0 Object Library, using code like:
Const acColorIndexAqua = 14
They are primarily used as arguments for the DoCmd.
ActionName statements. To display the complete list of them, open the Oject Browser (F2), select Access in the Project/Library list and select Globals in the classes list (or simply place your cursor anywhere in a standard module and press Ctrl-J.)
Each of the default object libraries referenced by an Access application has its own distinguishing intrinsic constant prefix, so you'll also see:
"
vb" - Visual Basic For Applications Object Library
"db" - Microsoft DAO 3.6 Object Library
"ad" - Microsoft ActiveX Data Objects 2.x Library
HTH,
Bob