§1. As a general rule, a conference of bishops includes those who preside over all the particular churches of the same nation, according to the norm of can. 450.
§2. If, however, in the judgment of the Apostolic See, having heard the diocesan bishops concerned, the circumstances of persons or things suggest it, a conference of bishops can be erected for a territory of lesser or greater area, so that it only includes either bishops of some particular churches constituted in a certain territory or those who preside over particular churches in different nations. It is for the Apostolic See to establish special norms for each of them.
Source: Code of Canon Law (Vatican.va)