In My activities section, you have access to all the tickets you have created, the tickets you are in CC and the tickets which have been created by users from your organization.
You have no access to tickets created by other organizations.
The tickets are sorted by date.
See below tickets you are the Requester - My requests tab:
Tickets you are in CC - Requests I'm in CC'd on tab:
Ticket of your organization - Organization requests tab :