There are two tables linked to each other via ID column. tbl1, tbl2
each table has a field called rowguid
the question is to retrieve fields 1, 2, 3 and sequencenumber
My question is to get the sequencenumber, is it referring to something like row_number()?

Yes basically something like this

SELECT col_list, row_number() over (order by some_col) as sequence_no 
from tbl1 as t1 inner join tb2 as t2 on t1.rowid=t2.rowid