Revit Guide Grids are useful to align views on a sheet with. Depending on the size of the sheet and the purpose of the grid, one may define these Guide Grids (System Families) with a certain overall size and a grid spacing. Their power lies in being able to align grids and levels to the grid lines, and that they appear exactly in the same position on each sheet. So a plan views can appear in exactly the same place with respect to the title blocks all the time, or an elevation may be perfectly aligned with a plan view.
I ran into an interesting situation the other day where I wanted to rename the Guide Grid. I had a look for the Guide Grid system family in the family browser, but it does not appear there. Neither are the Type properties accessible through the Type Selector. Instead, the properties are instance-based and act as a type property would. One has to manage the Guide Grids through the user interface. It is not possible to re name it either, but one may delete and add another.
If you need help adopting BIM or using Revit within your company, please contact Micrographics so I may be of assistance.