What should the summary count on an "active filter" button be representing?

Often, in facetted filtering experiences, on mobile, the filters are tucked offscreen. To indicate that the current view is being filtered, often the filter button is presented with a number indicating how many active filters are applied. ex: [ Filters (2) ]

This number is inconsistent from retailer to retailer. It can be either:

  • the number of selected filter values
  • the number of filter categories with filter values selected in them.

Amazon represents the number of filter categories selected:

  • Brand: Intel & AMD
  • Shipping: Free
  • Cores: 2, 4 & 8

Amazon

Walmart represents the number of filter values selected:

  • Brand: Apple & Sony
  • Shipping: Free
  • Colour: Grey, Pink, White

Walmart

As far as I'm concerned - the biggest usability gain comes from having anything indicating an active filter state - the exact count isn't that essential.

Pro Filter Category

  • Numbers past a certain point don't really matter
  • It may be more important to know the number of categories, rather than values
  • When you open the filter button, you tend to see categories - tap 3, see 3 selected things

Pro Filter Value

  • Every time the filter changes, the number is guaranteed to change
  • It's a simple sum and may match user mental models.

From my audit around the web, it's about 50/50 and I guess either one works, but is there any other arguments that can work in favour of one or the other approach?

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