It's the Lombardy region's name for what used to be ASL. The ATS plans and oversees public health in its area, while the actual care is delivered through ASST hospital trusts and clinics. If you live in Lombardy, the ATS is where some health paperwork lives.
If you move to Milan or anywhere else in Lombardy, you'll see ATS on health forms instead of ASL. Same idea, different name, same function for residents.
"When Petros moved from Sicily to Como, he went to the local ATS office to transfer his SSN registration. The clerk pulled his old file and assigned him a new family doctor in his new neighbourhood."
ATS does planning. The day-to-day care like family doctors, vaccinations, and outpatient services is delivered through ASST. If you need a GP, you'll usually deal with ASST, not ATS directly.
Official source
Regione Lombardia
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