Type A means it catches sinusoidal AC faults plus pulsating DC faults, which is the standard for most general-purpose circuits feeding electronics or variable-speed drives. The 16 A AC rating is its continuous current-carrying capacity; derate it in warmer cabinets — at 45 °C it's still good for 14.88 A, and at 70 °C it drops to 12.8 A. That derating curve matters if this lives in a hot panel next to contactors or drives.
Mounting position any, so you can lay it sideways in a shallow enclosure if needed.
Current-production part — Siemens still makes this 5SM2121-6. No official successor has been issued, and no end-of-life notice is on record.