No preposition, usually translated as “by ____” (or figuratively as a subject)
Used in poetry with any passive form, usually a participle
Used in prose ONLY with a passive periphrastic
Used in place of an ablative of agent in passive constructions only.
Examples:
Fēlīx est dicta sorōrī. (She was called happy by her sister)
Haec vōbīs prōvincia est dēfendenda. (This province is for you to defend)
Quibus est ā vōbīs cōnsulendum. (for whom you must consult)