When they added the Guide Grid feature to Revit they made it possible to snap to Grid intersections (and Reference Planes) through a viewport. This means we can move a viewport into the same location from one sheet to the next based on an agreed grid intersection such, "A1 will be what we use as a reference location to put our floor plan views on the sheet the same way."
They added this after many years of hearing users complain about views not lining up on sheets. Me personally, I got over it early. Seemed to me that far fewer sheets were affected by this concern than the number of sheets that weren't. I don't mind having the feature but it just never bothered me. I could get things close enough that nobody would notice, especially working with real paper in hand. Granted it is easier to tell in the digital world "flipping" pdf pages or comparing a CAD file overlay. For me though, it didn't really amount to a hill of beans in the job trailer. As a contractor in the past I was more worried about information actually being on the sheet at all to be worried about whether they stacked from sheet to sheet exactly.
Oops I digress...
In order to make this possible they needed to make Revit more sensitive to the contents of the viewport when using the Move Tool. Unfortunately Revit seems to think we live in an orthogonal world because the only Grids or Reference Planes the tool "sees" are orthogonal ones. No arcs, no grids at an angle, sorry Charlie. Now it isn't hard to place a couple orthogonal Reference Planes somewhere or to choose a different Grid intersection perhaps but it would be nice if Revit was more malleable, enough to let us pick any intersection of Grids or Reference Planes.
From Autodesk's WikiHelp:
Item 7. Snap to the crop regions or datums in the viewports and move them into alignment with the guide grid lines to specify a precise location on the sheet. (bold emphasis mine)
This means we can forget about the datum stuff (Grids/Reference Planes) and use the Crop Region. Of course the Crop Region has to be visible to snap to it and unless you are using a Scope Box to manage the Crop Region (to keep them consistent for many views) it isn't really the most reliable reference point to use either. Here's a visual aid, a short video discussing it too.