Security can limit access to layouts. You can also use Get(Device) and Get(SystemPlatform) to determine the device and what platform the user is using then go to the layout designed for that device and or platform.
A follow on question to your answer. The Get(Device) and Get(SystemPlatform), is this a script or in the security settings, and does this answer still apply if I am using Filemaker 14?
Get(Device) and Get(SystemPlatform) would be used in scripts. You would use script triggers such as onFirstWindowOpen then then go to the layout based on the device and or platform. See any of the starter solution.
Yes, this applies with FM 14.