It's the entry quotas set in the yearly Decreto Flussi, breaking down how many non-EU workers can enter Italy by sector, country, and permit type. The numbers cap how many nulla osta can be issued in each category.
If your sector or country isn't in the year's quote, the standard work route is closed for you. Some categories outside the quote (Blue Card, ICT, startup visa) bypass these caps.
Filling fast doesn't always mean instant rejection. Some applications wait in line for the system to process unused slots, but you can't count on that.
Official source
Ministero dell'Interno
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