> 
> Would someone like to tell me the relationships between biosboot.sym and mbr ?

mbr resides on mbr (first sector of disk). biosboot.sym resides mostly on
filesystem and few of it's code resides on the beginning of
netbsd bootable partition (code which loads the rest).

if your boot part begins at sector 0 no mbr is used (like my machine).

> Is that to say  mbr is contained in the biosboot.sym ?
> but I found that mbr of the disk is not refreshed with the following command:
> # installboot -f /usr/mdec/biosboot.sym /dev/rfd0a

mbr is installed with fdisk