§1. Within three months from the reception of episcopal consecration or if he has already been consecrated, from the canonical provision, a metropolitan is obliged to request the pallium from the Roman Pontiff either personally or through a proxy. The pallium signifies the power which the metropolitan, in communion with the Roman Church, has by law in his own province.
§2. A metropolitan can use the pallium according to the norm of liturgical laws within any church of the ecclesiastical province over which he presides, but not outside it, even if the diocesan bishop gives his assent.
§3. A metropolitan needs a new pallium if he is transferred to another metropolitan see.
Source: Code of Canon Law (Vatican.va)