This will largely depend as to whether the lateral movement is one that The FA has initiated or whether the Club in question has applied for consideration to be give as to a lateral movement.
If a lateral movement is deemed necessary this will more than likely be as a result of a Club being promoted or relegated to a given Step Level which is to be clearly placed within the boundary of a given League. In the example below, the marker in blue represents a Club who have been promoted or relegated to this Step Level. It is clear that they would form part of the membership of the 'yellow League' and would be allocated as such. The attention would then turn to which of the yellow markers should become either 'green' or 'purple' - i.e. be laterally moved - when taking all relevant factors into account, for example: travelling distances, travelling times, frequency the Club have been moved historically, whether or not they have previously been a member of a neighboring League:
If a Club has requested a lateral movement their reasoning will be specific to that application. In general terms, Clubs will request lateral movement based on the driving times and/or distances involved with their current and prospective Leagues or that they had previously been laterally moved without applying and they want to be considered for a movement back again.
It should be noted that any location of a Club is based on the Club's home ground and not where the player-base is situated.