end man on LOS is eligible; if the wr lines up "on" the line of scrimmage, the TE is ineligible. WR needs to line up "off" (basically considered in the backfield even though he's split wide) so the TE can go out for a pass. This is considered the "flanker."

If there is no TE to that side of the formation, the wr needs to line up "on" and cover the tackle, who is ineligible by position and by number (unless he reports). This is the "split end."