Showing Held Seats on United

Does anyone know what the "held" seats mean? I have tried even booking a space positive ticket but United says there are no seats left. Can it be a weight restricted flight? Flights within the next few days show 0 held seats but all flights further out show 6 held seats. Thanks for your help!

https://employeerespss.coair.com/emp...ges/spacer.gifhttps://employeerespss.coair.com/emp...ges/spacer.gif
XXX-XXX	Avail.	Capacity	Auth.	Booked	RSB	Held	PS	SA	Group
F/C/Y:https://employeerespss.coair.com/emp...ges/spacer.gif	0/0/-1	16/0/144	16/0/145	16/0/139	0/0/0	0/0/6	0/0/0	0/0/1	0/0/0
Totals:https://employeerespss.coair.com/emp...ges/spacer.gif	-1	160	161	155	0	6	0	1	0

I've not seen that before but I'm relatively new to UA. Maybe Spongebue cal tell us?

Thanks for trying :) This is really my only hope to get to ANC as all other flights are oversold on all airlines from any city... haha