[Translator Reform] Core Grids: What constitutes the use of a 21x21 grid as opposed to a 31x31 grid?

NOTE: This answer relates to a rule proposed by REC and does not reflect current regulations in effect:

In the post-2003 processes, the 21x21 grid was used in areas where the population in the area outside of the 21x21 grid but inside the 31x31 grid was not substantially dense. For our current study, we are only applying the 21x21 grids in areas where the FCC applied them in the post-2003 procedures.