A 100 A frame means it carries 100 A continuously at 40 °C, 45 °C, and 50 °C without derating; at 55 °C it still holds 96 A, and at 70 °C it delivers 90 A. Breaking capacity is the other headline number: 121 kA at 240 V, 75.6 kA at 415 V, 52.5 kA at 440 V, and 11.9 kA at 690 V. That is a high-interrupting rating for a 100 A frame, suited for transformer-secondaries or high-fault industrial panels. That makes it a straightforward line-protection breaker for cable and busbar protection, not a motor-protection or selective device.
The 76.2 mm width is a 3-pole frame — three module-widths at roughly 25.4 mm per pole — so it occupies three adjacent 45 mm DIN spaces in a typical distribution board.