Where should I stand in open play, at lineouts and at scrums?

World Rugby's assistant referee guidance (the referee may ask for different positions):

Open play: leading and trailing

• Leading (play coming towards your touchline): watch the ball-carrier for foul play and judge touch on your side.
• Trailing (play moving away): stay slightly behind play and watch rucks and mauls breaking up. When the ball is kicked downfield, note where it was kicked from, in case it goes directly into touch or dead, or there is a late charge on the kicker; stay on that mark if needed.

Lineout

• On the lineout side: stay at the line of touch until the ball is thrown in.
• On the opposite side: stand on the non-throwing team's defensive line, 10 m back from the line of touch.

Scrum

• Opposite the referee: in line with the tunnel.
• Behind the referee: on the offside line of the team not throwing in, 5 m back from the hindmost foot.