Short answers, pulled from the story.
A contubernium was the smallest organized unit of soldiers in the Roman Army, composed of eight legionaries who shared a tent, equipment, and a pack mule. The Latin word means "tenting-together." Ten contubernia formed a centuria of one hundred men commanded by a centurion.
The contubernium was used to maintain morale and group solidarity by keeping soldiers in close contact with each other. It had no tactical significance in Roman military organization or battle, distinguishing it from the building blocks of most modern armies.
A contubernium was led by a decanus, also called the caput contubernii, a title attested at least by the 4th century CE. The decanus was roughly equivalent to a junior non-commissioned officer and was most likely the longest-serving legionary in the group, appointed from within rather than assigned from outside.
A contubernium consisted of eight legionaries. Two auxiliary servants were also assigned to each contubernium, making a total of ten personnel per group. The members of a contubernium were collectively called contubernales.
Soldiers of a contubernium shared a tent, equipment, and a mule for transporting supplies. They could also be rewarded or punished together as a unit, including under the collective penalty known as decimation.
Two auxiliary servants assigned to each contubernium were responsible for caring for the group's pack mule and ensuring legionaries had water during the march. Some may have possessed specialist skills such as blacksmithing or carpentry, though in many cases they likely served as grooms and general servants.