What's difference between them?
Check the help:
FileMaker Pro 15 Help
SystemPlatform is the older of the two functions and gives you info on the OS, but not the device. Get(Device) has only been around since FM13 and can give you more granular info especially for what type of iOS device.
If you only care about the OS (mac / windows / ios / webdirect ) then use Get(SystemPlatform), if you want to know what device the user is using then use Get(Device)
Get(Device) tells you if what device is used regardless of the platform. For example FM Go on iPad would return the same as WebDirect on iPad.
Get(SystemPlatform) tells you more about the platform used to run your solution. In above exampe it would therefore return different results.
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