The 1.5SMC6.8A: The 5.8 V reverse standoff is the working voltage the TVS sees in normal operation — the diode sits idle below this, leaking only microamps. For a nominal 5 V rail, that window — standoff to clamping — is tight enough to clip a fast transient without exceeding what downstream silicon tolerates. The 1500 W peak pulse power rating defines the energy the device can absorb in that 10/1000 µs waveform without degradation. This is the IEC 61000-4-2 class of transient — ESD and switching surge — that the 1.5SMC series is sized for in board-level protection schemes.
The part is listed Active by Littelfuse. Confirm the BOM line voltage matches 5.8 V standoff before committing.