• More onboard RAM, allowing e.g. expanding Atari to 1088kB (in all scenarios, not only ATR, including 16kB stock Atari 600XL) or provide 4MB AXLON compatible mem expansion (PBI or ECI cable required)
• built-in battery backed RTC with SDX driver
• Optional DAC cable allows stereo POKEY/COVOX emulation (PBI or ECI cable required)

You need ECI or PBI cable to make use of any extra features. Using it on a modified machine (3rd party internal memory expansion, U1MB) is not recommended.