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No. Of Productions
From The re-PSUPedia
A Synthesis Board can only be used in so many Synthesis attempts. There will be a numerical value (eg '3/5') to show the number of available Synthesis attempts left on that Board compared to the maximum amount of Synthesis attempts possible.
This number will go down by one with each Synthesis attempt, regardless of whether it succeeds or fails. When the Board has no more productions left (i.e. 0/x) it will disappear, and you will need to find or buy another one to continue making the item.
In general, rare items have less uses than common items.
