Pi 2 requires a permanent SD card as its built-in bootloader does not support network booting. It does not support USB booting. Pi 3 B has support for both US

Pi 2 requires a permanent SD card as its built-in bootloader does not support network booting. It does not support USB booting. Pi 3 B has support for both USB and network booting in its bootloader, but they are disabled by default. They are turned on by modifying some OTP configuration bits, which is a permanent change. Once set the SD card is no longer required. Pi 3 B+ has network and USB booting enabled by default. Presumably Pi 3 A+ will support USB booting out of the box, who knows it it'll also support USB ethernet adapters for PXE.