VCMDB(Volume Configuration Management Database) stores access confiurations that are used to grant  host access to logical devices in a Symmetrix storage array.The VCM database resides on a special system resource logical device,referred to as the VCMDB device,on each Symmetrix Device.

Host and storage WorldWide Names, SID Lock and Volume Visibility settings, Native logical device data, such as the front-end directors and storage ports to which they are mapped.

DMX 4

  Minimum  Max
Disk Directors 2  8 8 ports per Director    
Disk Channels 16  64 each drive supported by 2 disk channels for redundancy

Protocol     usable System Ports  Channel Director
4 Gb/ FC host/SAN ports   2-64    1-8 per FC Director
4Gb/s FC remote replication ports 2-8    1-4 FC Director
   

Mapping:Making it available to the outside world from EMC point of veiw.

Masking:Making the device available to specific host.(If you are mixing multiple hosts in FA you may want some devices available to only to specific host(s).

Mapping is Mapping the devices to the FA on the DMX.
MAsking is mapping the WWN of the host to the FA and Devices

Mapping is withn the DMX to designate which Fibre Adapters the LUN will use.Masking is outside the DMX, creating the path from the Fibre Adapter to the HBA on the host.