Select statement in from clause

Hi i found the next select statement and i was wondering what this select statement in the from clause does ? It is the first time i see this.

Thnx

Dennis

==================SQL===========================

select	rpad(nvl(header.bill_number,' '),8)||
	nvl(substr(to_char(header.customer_number,'0999'),2),'    ')||
	rpad(nvl(header.title,' '),50)||
	nvl(to_char(header.date,'ddmmyy'),'      ')||
	substr(to_char((nvl(times.manuf_time, 0) * 10000), '0999999'), 2)||
	substr(to_char((nvl(line.aantal, 0) * 1000000), '0999999'), 2)||
	rpad(substr(nvl(customer.customer_name, ' '), 1, 30), 30)||
        rpad(substr(nvl(header.pcode, ' '),1,10), 10)
from	bills header


,	detail line

,	(	select	a.bill_number

		,	(nvl(sum(b.unit_time), 0)/60)*1.1 manuf_time
		from	detail a
		,	article b
		where	a.part_number = b.part_number
		group
		by	a.bill_number
	) times


,	customer

where   header.bill_number = times.bill_number(+)
and	header.bill_number = line.bill_number(+)
and	line.part_number = '5000'
and	header.customer_number = customer.customer_number(+)
and	not exists
	(	select	1
		from	bills_sended sended
		where	sended.bill_number = header.bill_number
	);