Hello,
If you have VTL in the environment, capacity based licence may be 
beneficial, but you are not tied to it, there may be additional limitation 
on unlimited capacity based on how many instances of VTL can you create to 
use that licence, i think currently it's max 4.If  you already own 
unlimited slot licence and let VTL do the cloning ( in case of NetApp VTL) 
you may not need any additional licence.
The drawback of unlimited licence for physical lib in certain VTL 
integrated environment  that it  could become limiting factor, depending 
on your design, i.e if you want to assign a separate instance of VTL to SN 
or extra master server.
You may still need actual physical licence depending on what is your 
cloning method.
HTH

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