clear itab，僅清空HEADER LINE，對內表數據存儲空間不影響，保留內存區。 refresh itab，不清空HEADER LINE，清除內表數據存儲空間，但保存內存區。 free itab，不清空HEADER LINE，清空內表數據存儲空間。
clear itab[],清空內表數據存儲空間里的內容。
例子：
REPORT ZZZZZZZZZ.
types:begin of head, mantr(10) type c, id(10) type c, end of head. data gt_head type table of head with header line.
gt_head-mantr = 'wo'. gt_head-id = '00001'. append gt_head.
gt_head-mantr = 'you'. gt_head-id = '00002'. append gt_head.
free gt_head. write:gt_head.
loop at gt_head. write: / gt_head. endloop.
gt_head-mantr = 'he'. gt_head-id = '00003'. append gt_head. write:gt_head.