The SMAJ5.0CA-13: That 400 W rating is the energy the part can absorb in a single surge event — above that, the junction fails short. For a 5 V nominal rail, the reverse standoff voltage of 5 V means it sits across the supply without conducting until a transient pushes the line above 6.4 V (the breakdown minimum).
SMA package and board integration
Housed in the standard DO-214AC (SMA) surface-mount package, the SMAJ5.0CA-13 mates with a footprint that is common across the SMAJ family — same pad layout as the unidirectional variants, so a single PCB land pattern serves both.