From Halopedia, the Halo wiki
The messenger satchel is a UNSC canvas transport bag. It was rated for storage and transport of UNSC chemical and plastic munitions as well as Covenant Plasma Grenades and Spike Grenades.
Its contents are legally protected by the Martial Circumstance Rules and the Official Secrets Act of 2550. Penalty for unauthorized dissemination or distribution of the contents is summary execution by a UNSC authorized firing squad.