CircuitStudio supports Altium Designer PCB Projects (*.PrjPcb) and will open any Altium Designer Schematic (*.SchDoc), so it can also be used as a front end for Altium Designer and other Altium products.
Note: CircuitStudio has its own PCB file format (*.CSPcbDoc). Use File >> Import to open CircuitMaker (.CMpcbdoc) and Altium Designer binary 5.0 (.PcbDoc) files.
The PCB file must be exported from Altium Designer as a specific binary version: In Altium Designer, with your PCB open. File >> Save Copy As, set the Save as type: to PCB 5.0 Binary File (*.PcbDoc).
The 6.0 format is not yet supported in CircuitStudio, even though it is listed.
See this article for more details of compatibility with Altium Designer.