One issue I wanted to call out regarding this information in the cited Knowledge Base article:
There are a few exceptions to the above. For example, an exception is that emails sent and calls made from any object record will also be associated to the relevant contacts that were being emailed or called.
This is not accurate if users are on the mobile app. Sales users actually aren’t able to send an email from a deal at all on the (iOS) mobile app -- the option is simply not there. If they send an email to a contact who has an associated deal, the sales user cannot even edit the defaulted assocations from the email (as they can in the web app) to include the deal.