Clarify what we mean by “ancillary” (#459)

Co-authored-by: Theresa O'Connor <hober@apple.com>
Co-authored-by: Tantek Çelik <46418+tantek@users.noreply.github.com>
Co-authored-by: fantasai <fantasai.bugs@inkedblade.net>

Author: 4 people

index.html

@@ -1304,10 +1304,19 @@
 
 ### Ancillary uses
 
-[=Sites=] sometimes use data in ways that aren't needed for the user's immediate
+[=Sites=] sometimes use data in ways that aren't needed for the user's primary
 goals. For example, they might bill advertisers, measure site performance, or
-tell developers about bugs. These uses are known as <dfn data-lt="ancillary
-use">ancillary uses</dfn>.
+tell developers about bugs.
+These are examples of [=ancillary use=] of [=data=].
+
+An <dfn>ancillary use</dfn> is any case where [=data processing=]
+provides direct benefit primarily to [=actors=]
+other than the [=person=] who is the subject of that [=data=].
+Ancillary uses of [=personal data=] might provide benefit to that [=person=],
+but any benefit is indirect.
+For example, being able to charge advertisers provides benefit in that
+the site can sustain its business and remain available for future interactions,
+but this benefit is primarily realized by the site's owners.
 
 [=Sites=] can get the data they want for [=ancillary uses=] from a variety of places: