Australia AIP ENR 1.5 34 explains all.
Basically if the weather at your destination falls below the Alternate / Special Alternate minima published on the destination approach plate during your period of suitability then you must nominate an alternate.
Special Alternate minima are published at certain aerodromes for the use of aircraft with the DUAL ILS / VOR approach capability i.e. they have two ILS / VOR receivers installed and serviceable.
SLAM (Special Alternate Minima) is NOT available for use if:
Local Metar / Speci forcasting unavailable (as notified by NOTAM)
An aerodrome control service is not available (as notified by NOTAM)
There is a protracted unservicability of any VHF approach aid of more than 7 days
So if a SLAM is published and you don't have any of the above NOTAM'ed conditions and your aircraft has the capability then you can use the SLAM minima.
