RBX.lua.Selection (Service)
From Legacy Roblox Wiki
The Selection object.

Functions
In addition to the global functions, the Selection also has these functions: # indicates a locked function. They can only be used in Studio.
Properties
The Selection object only incorporates the global properties.
Events
The Selection object only incorporates the global events.
Usage
In Studio/Edit mode, first select some bricks then:
local cf = CFrame.Angles( 0,math.pi,0 ) for _,v in pairs(game.Selection:Get()) do v.CFrame = cf * v.CFrame end
Also, you can set which objects are selected with the Set method. So then you can do stuff like select all objects with a certain name, then quickly edit their properties as desired.
local select = {} for _,v in pairs(workspace:GetChildren()) do if v.Name = "Part" then table.insert(select,v) end end game.Selection:Set(select)