§1. Subreption, or concealment of the truth, prevents the validity of a rescript if in the request those things were not expressed which according to law, style, and canonical practice must be expressed for validity, unless it is a rescript of favor which is given motu proprio.
§2. Obreption, or a statement of falsehood, also prevents the validity of a rescript if not even one proposed motivating reason is true.
§3. The motivating reason in rescripts for which there is no executor must be true at the time when the rescript is given; in others, at the time of execution.
Source: Code of Canon Law (Vatican.va)