privacy

for destiny 2 operations, there are 2 different levels of privacy:

• things you can do anonymously (meaning "without a user account", but you still need an API key)
• things you can do authenticated as a user, involving that user's account (some examples here)

there's no middle option, "things you can do as a user, involving other people's accounts".
things are either public, or locked to only the user involved.
so there's no need to be logged in as X unless it's for information specific to X's account.

what sorts of things are private?

• inventory changes
  • moving items around/pulling from postmaster/equipping
  • plugging operations
  • locking/unlocking

what sorts of things may be private?

• activity history
• collections/triumphs data
• non-equipped inventory

what sorts of things are always public?

• account information
  • Bungie name
  • information about what other accounts are linked/cross-saved
• a player/character's participation entry in a pgcr
• an account's characters
• each character's equipped items

things that may be private are configured by each Bungie.net user at
https://www.bungie.net/7/en/User/Account/Privacy