Trigger - Unsure On How To Update A New Row Rather Than Whole Table

I have created a simple trigger that updates a table called documents in my database. When a row has been inserted it will change the field form_type to 'cabinet A' if a document_type is equal to 'form'.

Im fairly new to triggers and I��m uncertain whether this trigger is updating the whole table or the new row? I only want the new row to be updated as the table is fairly large.

create trigger file_documents
on documents_table
after insert
as
begin
update documents_table
set cabinet = 'Cabinet A'
where document_type = 'FORM' end