It's just a way to get around those pesky conditional phrases in a TAF (tempo, prob30, becmg, etc).
The main body of the TAF (FM group) needs to be at or above landing mins, but there can be a conditional phrase that drops the vis to 1/2 of the required landing mins.
For example...Lets say your destination requires 1/2sm vis to be dispatched. The main body of the TAF is calling for 1/2sm or better, however there is a tempo during your ETA for 1/4sm. You can be released under exemption 3585.
You will require 2 alternates. The first can meet the same criteria as the destination (main body at or above alt mins, conditional 1/2 of alt mins), but the second alternate has to be at or above alt mins.