How can I apply an attribute to all companies, deals, or people?

There are a limited number of bulk actions that you can perform from the list view pages, depending on which you're looking at:

People: Add task, reassign ownership, set status, add or remove tags, delete, or create a deal. If you have the MailChimp integration, you can export to MailChimp, and if you are on our Accelerator platform, you can email or a campaign.

Companies: Add task, reassign ownership (if Executive), delete.