Let me make sure I understand this line by line and have each right so far...

First, isn't the line "WHERE Transaction_Type = 'From Bulk Assignment'"

correctly referenceing the INSERTED with no additional qualifiers for this line needed?

USE [TrackIT]

GO

CREATE TRIGGER [dbo].[trDecrementBulkPurchases]

ON [dbo].[tblTransactions]

AFTER INSERT

AS

BEGIN

UPDATE [TrackIT].[dbo].[tblBulkPurchases]

SET [Quantity_Remaining] = [Quantity_Remaining] - 1

WHERE Transaction_Type = 'From Bulk Assignment'

AND PO_Number IN (SELECT [PO_Number] FROM [tblBulkPurchases])

END

GO

WHERE Transaction_Type = 'From Bulk Assignment'