Attachments are uploads you make against an incident, exercise, member, qualification, or item of equipment, etc. they are files 'attached' to the item. They appear in context, on the item's page, or in its sidebar.
Any user with the proper granular permissions will be able to upload attachments. For example a 'Member with Draft Activity' permission add-on, will be able to upload attachments to draft activities even if they don't have 'Member+ with Documents' permission.