The Siemens 5SL6620-7YA is a SENTRON miniature circuit breaker (MCB) with a 3-pole+N (3P+N) design, meaning it switches and protects all three phases plus the neutral conductor in a single 4-module-width (72 mm) package. It is rated 20 A at 400 V AC, 50/60 Hz, with a C-curve trip characteristic — so it holds through brief inrush up to 5–10× rated current (typical for motor starting or capacitive loads) and trips faster on sustained overloads. The breaking capacity is 6 kA according to EN 60898, which tells you it can safely interrupt a fault current up to 6,000 A at 400 V without welding its contacts or rupturing the case. It carries pollution degree 2 (non-condensing, typical of a clean indoor panel) and overvoltage category III (distribution-board level, not sub-circuit).
If your panel was laid out for a 3-pole MCB (like the 5SL4363-7), you'll need an extra 18 mm (1 MW) of DIN-rail space to accommodate the 4-MW width of this 3P+N unit. The higher 10 kA breaking capacity of the 5SL4363-7 may also matter if your fault level is above 6 kA; otherwise the 5SL6620-7YA is a direct functional drop-in for the same 400 V AC, C-curve, 20 A application. The DC rating is 72 V maximum — useful if the same panel also feeds a DC bus, but note the DC breaking capacity is not separately listed; for DC circuits above 72 V, a dedicated DC-rated MCB is needed.