Is there any limitation on the format of AppVersion

Hi Lior.
It has to be a string that shouldn't be too long (obviously it would be a waste of bandwidth to send around a thousands of characters long version string, although it would work). Clients that are compatible to each other should use the same version string and clients that are incompatible to each other should use different version strings, so that they will not get matched with each other (each different version is treated by Photon as if it would be a completely different app). So these version strings should not duplicate your actual game version, but the network version of your game, which you should only change, if you do not want clients with the new version to be matched with ones that are still on an older version of the game.