I have no direct knowledge but: SSI could be an acronym for "Società Speleologica Italiana". In Italy we have a national registry and many regional registries (with different codes)
https://speleo.it/catastogrotte/wish

cave:ref:it:SSI=* these codes are not SSI but are regional codes of Veneto (FSV "Federazione Speleologica Veneta")
cave:ref:it:CSL="2006" should be cave:ref:it:SSI="LO2006"

these nodes seem like simple tests

Hi user_5359, thanks for checking.
I have modified many caves in my region and followed this wiki:
osm.wiki/Cave#Cave:ref_and_variants
Probably the correct form is with "IT",
as soon as possible, update all data with uppercase characters, thank you

Hi! I'm working on the caves in my region, thank you for checking. I followed this scheme: osm.wiki/Cave#Cave:ref_and_variants
FST = "Federazione Speleologica Toscana"

per la tua segnalazione, non potrebbe essere utilizzato "amenity=social_centre" ?

I'm sorry for the change, I had to ask for information. In the whole world there were only 16 entities, the key had appeared on a quality control portal; documentation should be written to explain this key.
payment:*=*