I was Quizzed with Ranks in DAX and during my exploration few intresting things were found. Here are my findings on how ALL and ALLExcept function behaves with contexts !
Here is the structure of the "Doctors" Table : As you can observer the Spec and Hospital are from the same table from which i have created the measures which makes it intresting !
The measures were sliced against Hospital , Spec column to observe the behaviour :
Keep close attention to the the highlighted section
As you can notice from the section :
fig 1. Hospital filter context doesnt change ALLEXceptSpec measure as it will only change when the filter is on "Doctors[Spec]" Column .
fig 2. Spec filter context change ALLEXceptSpec measure as said above and ALLSpec measure ignores the spec filter .
Here are few more links on the same :