Thats a vanilla description of several crusade units. However, I think after CA wrote it, they realized that they couldn't remove a unit after the objective was taken or so.
As with any crusade units, it cannot be retrained (I had a lengthy thread about retraining mercenary mechanics in the modding wiki), so you can at best use it to replenish other depleted units of the same type.

Great Cross is a light artillery. Light artilleries do not shoot, they are basically standards in M2TW, meaning their presence (if manned) give nearby units a morale boost.