Nested Transaction in SQL Server

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SQL

server

Hi Experts
I Have Stored Procedure and apply transaction on that S.P in between of S.P There is Trigger fire in S.P and Trigger also have transaction apply and if there is any exception or error occure in Trigger then S.P "roll back" or "Commit" the transaction.

E.g

Create S.P abc
as
Begin Transaction
Try
select * From xyz
Update rst set a='1' where r='z' --when update trigger fire on rst table update trigger
select * from rst
Commit
catch
Rollback
end
End Transaction

Create Trigger nc on rst
update
Begin Transaction
Try
-----------any thing logic here-
Commit
catch
Rollback 
end
end Transaction

-- But in My Trigger Exception Occurs then next statement in S.P is Run or Whole S.P Rollback
--Or Another Case Suppose My Trigger Run Successfully without error and error occur in S.P then But Happen.