Short answers, pulled from the story.
The word sacramentum meant a thing pledged as a sacred bond that rendered the swearer sacer or given to the gods. This status applied negatively if the person violated the oath and implied an underlying act of consecration known as sacratio.
A sacramentum established a direct relation between the person swearing and the gods themselves while iusiurandum functioned as an oath of good faith within the human community. The military oath rendered soldiers sacer placing them under divine jurisdiction which explained why harsher penalties applied to military personnel compared to civilians.
By the third century the oath was administered annually on January three according to a calendar of state ritual discovered at Dura-Europos. The Feriale Duranum dates to the reign of Severus Alexander between two hundred twenty-two and two hundred thirty-five AD.
The sacred status of soldiers justified execution and decation because violations were treated as offenses against the gods rather than mere crimes. Decimation involved killing one out of every ten men in a unit that failed and surviving legionaries often renewed their oath after such punishments occurred.
Tertullian condemned any Christian soldier's willingness to swear the sacramentum oath arguing that baptism was the only sacrament a Christian should observe. Christians serving in the military produced a number of soldier-martyrs during the later empire because the military oath demanded loyalty incompatible with Christian baptismal vows.