I would say there might be a slim chance that your company & customer might let you walk over to Taco bell down the street while they load/unload you. If that is the case and your company has given you permission "prior" to dispatch that you can log off duty while under that load then sure you can log that as off duty. I just see that being a rare occassion the customer # 1 would let you leave the premises.
Be honest with yourselves and the answers come easy!
When you are at a customer you get there and sometimes wait, when you are waiting the best thing to do is go into the sleeper and read a book, relax whatever! IF you can't and you have to sit up there and "wait" that means you are on-duty. If you can go to the sleeper then log sleeper. I know many customers do not want you around, they say get out of here! Well what is a driver to do? Go to your sleeper.
If DOT ask tell him/her they didn't want me around so I went into my sleeper and laid down to relax. What can he/she say to you? Oh you logged the correct duty status in that case
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Now if you are assisting with that load then you better be logging line 4 the whole time!
Also it's common sense it's going to take @ least 15 minutes to deal with that load/unload, so log a minimum of 15 minutes, but you SHOULD be logging however long it takes you to deal with it. If it takes you 1 hour, then log 1 hour. It stinks I know but it's how the DOT wants you to do it and sometimes they can figure it out
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Good luck