Selective deletion could be either executed by DELETE statement, or by Copy&Rename deletion procedure (that is, select the data that is not to be deleted in a temporary table, to delete the original table and then to rename the temporary table accordingly).
You can choose one of the approaches to achieve a better performance. This is controled by RSADMIN parameter: RSDRD_SD_710_THRESH (or BW_SELDEL_PERC_THRES), RSDRD_SD_710_PAKSIZE (or BW_SELDEL70_PAKDELROWS), RSDRD_SD_710_PARDEG
More details, read notes:
792435 Selective deletion: Using temporary tables
1587690 Problems with selective deletion in packages
1759583 BW data deletion with adjustable package size