FIXED names the exact dimensions. INCLUDE and EXCLUDE start from the dimensions in the view and adjust them.
{ INCLUDE [Order ID] : SUM([Sales]) } computes at the view's level plus Order ID: one total per order. Average it in the view and you get the average order value for each mark.{ EXCLUDE [Category] : SUM([Sales]) } computes at the view's level minus Category. In a view of Region and Category, that's each region's total, repeated on every category.
EXCLUDE always replicates values: every mark of the excluded dimension shows the same number, its parent's total. That's exactly what you want for a share, SUM([Sales]) / SUM({ EXCLUDE [Category] : SUM([Sales]) }), and confusing if you put it on an axis by itself.
Unlike FIXED, INCLUDE and EXCLUDE are computed after dimension filters, and they change when you change the view.