Context
Some say that once you have mastered the concept of context in DAX, you have mastered
DAX. This is mostly true, and if you have indeed mastered context, you are a long way up
the learning curve.
Context can seem a little daunting at first, but once you understand the effect that
types of context have on calculations, hopefully DAX starts to make more sense. Context
is how DAX applies layers of filtering to tables used in your calculations so that they
return results that are relevant for every value.
Most context is automatic, but some context allows you to control the underlying
data passed to your DAX calculations. This chapter mostly uses pivot tables to describe
the order and effect of the different types of context. Other visuals and charts apply
the context logic the same way pivot tables do, but I think pivot tables do a good job of
showing the effect.
There are two types of context in DAX: filter context and row context. Depending on
your calculation, one or both can apply at the same time to affect the result of your DAX
calculation.
Context is the layer of filtering that is applied to calculations, often dynamically
to produce a result specific to every value in a pivot table or visual, including row and
column totals.
Filter Context
Filter context is a set of column-based filters applied on the fly to the underlying data
for every DAX calculation. It is helpful to think of every value cell in a pivot table as its
own separate computation. Each value cell treats its measure as a function and passes
relevant filters to the function to be applied to the calculation
