How to read device layer table？

Hi all:
I wish to know how many layers of a devices, so I review the design truth table,
but there are three notes on each layer for every device, could anyone help me to know the it's real meaning?
ex:
              N_Well      DIFFUSION    POLY      N+     P+
NMOS         X                ．                ．            ．         X

1.  "．" means digitized area
     ==>does its mean the device is needed this layer?
2.  "X"
means non-digitized area
     ==>does its mean the device isn't needed
layer this layer?
3.  "DC" means don't care.
     ==>does its mean the device isn't needed
layer this layer?

Thanks for your reply.

mpig

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