Snooky's response is clearly knowledgeable but I'm not sure how universal his comments are.
In the UK, although it's hard to find, the rules are that runways have to be notified (by which I presume that means NOTAM'd or in the AIP) as 'liable to to be slippery
when wet' when routine testing shows that friction level has fallen below a specific level. The actual value varies depending on the friction testing equipment used and the values reported by the equipment also vary with other factors such as speed so I don't think the numbers produced are absolute but rather useful relative data generated under closely controlled conditions.
As I recall, ATC are not allowed to pass mu-meter readings from wet or slushy runways (instead we give surface conditions and braking action reported by pilots) - presumably because of the differing numbers that friction testing equipment comes up with in those conditions.
The above comes from an information notice issued to airport operators by the CAA and (the ATC bit) from what I remember from winter ops. It may be different outside the UK.
take ECAM action's original post said the the runway was reported as 'slippery when wet' - unless this was actually passed/broadcast as 'liable to be slippery etc' it seems to me that the statement is incredibly confusing and misleading.