From Halopedia, the Halo wiki
"Soft-seal" technology, used to create a "soft-seal" entrance, is a technology employed by the UNSC to help facilitate boarding actions. It was created and perfected during the later stages of Operation: TREBUCHET, during the Insurrection. Soft-seals can be deployed from D77-TC Pelican dropships, connecting the Pelican's troop bay to the outer hull of a given spacecraft. The seal creates an airtight fixture, allowing teams of boarding troops to then cut their way into the target spacecraft and launch a boarding action without risking depressurisation.
Soft-seals were used by Fire Team Lima in 2531 during their mission to enact the self-destruct of the UNSC Prophecy, a Marathon-class heavy cruiser which had been damaged during the Human-Covenant War. The team, led by Sergeant John Forge, used the soft-seal to cut their way into the Prophecy and engage the Kig-Yar who had begun to raid the ship.
List of appearances