The Siemens 5SJ3125-7 is a SENTRON 5SJ3 series miniature circuit breaker — a 1-pole unit with a C tripping characteristic, rated 15 A at 240 V AC, 50 Hz. Rated for AC/DC operation, overvoltage category 3, energy limiting class 3 — meaning it limits let-through energy enough to protect downstream equipment in a standard residential or light-commercial installation.
15 A at 240 V AC is the continuous current rating — this breaker will carry 15 A indefinitely without tripping under normal conditions. The C-curve means magnetic trip occurs between 5× and 10× rated current (75–150 A), so it's sized for loads with a moderate inrush that would nuisance-trip a B-curve but that don't need the 10–20× range of a D-curve. IP20 with connected conductors means the terminals are finger-safe only when wired — the breaker itself is for dry indoor panels, not washdown or outdoor enclosures.
Snap-on DIN-rail mount, 1-pole width (roughly 18 mm per pole). The 70 mm depth leaves clearance for wiring behind the rail. Supply can come from either end — the position of the power supply cord is any — which gives flexibility in busbar layout.