What are hidden folders in Exchange user mailboxes and how are they created?

Hidden folders are folders where the extended property `PidTagAttributeHidden` (0x10F4000B) is set to `true`. They remain invisible to users when accessing mailboxes via OWA or Outlook. You can create a folder under any mailbox folder (like Inbox) and set this property using EWS Managed API or SOAP XML to hide it from normal views.