use of required free mb in v$asm_diskgroup

Hello,

Please help me understand the use of   "required_mirror_free_mb" here.

SQL> select name,total_mb,free_mb,required_mirror_free_mb from v$asm_diskgroup;

NAME                             TOTAL_MB    FREE_MB REQUIRED_MIRROR_FREE_MB
------------------------------ ---------- ---------- -----------------------
DATA_QR01                           21312      10580                    7104
DBFS_DG                              5760       4392                    1920
RECO_QR01                           28800      27604                    9600

Best regards,

Vishal