The 1200 A frame means the breaker body is sized for that interrupting capacity, not the continuous rating — you get a physically large frame for a 100 A load, which gives you headroom on the interrupting side. Interrupting capacity is the headline here: 187 kA at 240 V, 121 kA at 415 V and 440 V, 75.6 kA at 500 V, and 3.7 kA at 690 V. The sharp drop at 690 V (3.7 kA) is typical for a 3-pole MCCB on a line-to-line 690 V system; the arc extinction physics limit it. The electronic trip unit is an ETU350 — that's a basic thermal-magnetic replacement with adjustable long-time and short-time pickup, plus instantaneous. It's a plain line-protection breaker: set the dials, wire it in, and it clears faults.
That 105 mm width is a standard 3-pole MCCB footprint for the SENTRON 3VA2 family — it occupies three 35 mm DIN spaces (3 x 35 mm = 105 mm). The 86 mm depth means it projects less than 90 mm from the DIN rail, leaving room for wiring gutters or a shallow enclosure.