tmp : location for temporary files written by the server
And on every reboot, a new tmp will be created and the deployed application is unzipped in that folder.
I think this behaviour must not affect your application's data, you may check out with your application if you can append your properties to previously saved ones...
If you use file IO this might have problems as the JavaEE spec does not allow this.
But maybe you can use some jboss environment variables to store it somewhere in the conf folder , in any case this is specific to the AS version.