In My activities section, you have access to:
-> All the tickets you have created under Requests tab, then My Requests
-> The tickets you are in CC under Requests tab, then Requests I'm in CC'd on
-> The tickets which have been created by users from your organization, under Requests tab, then Organization requests
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 :