Odoo helpdesk support ticket:

-Type tycket  --> This is for information purpose and it can be category of your ticket... for example like ticket by client, ticket by department, ticket by product, tickets related to bug, ticket related to project.. etc etc.. you can create any type of ticket types based on your business situation... 

-Type subjects --> This is kind of sub category of your ticket or you can use it as categorise your ticket name to group all the related tickets from client or by project.. For example Bug in accounting module is one ticket and Bug in Finance module is other ticket then you can categorise both in one subject called Finance.

- Allow users  --> All support users set on this allowed users field can access / update tickets.. So it is like allowing multiple support users to work on same ticket.

See Odoo app here