SPI is a bit more complex then described below.
SPI APPS can be used either through an Android phone connected through Bluetooth, or an iPhone connected through USB. If the system has both an Android connected by BT, and an iPhone connected with USB, the APPS screen will ask the user which device should be used.
SPI is more limited by the account associated with the unit rather than the number of devices. Any valid phone can be used with the APPS feature as long as the correct account is signed in.