They will pack absolutely everything - and yes, I mean everything! That includes rubbish in your kitchen bin or food waste in the compost bin if you have one!
We have always used a full packing service and we generally packed valuables ourselves (jewellery, guitar, documents, passports, anything in the filing cabinet etc) and then left the rest to the packers.
When moving within the U.K. I have done my books myself as I wanted them to stay in rough order for the bookcases. We have about 1500 books in the house though and I just didn’t have the energy to re-sort them when unpacking. Couldn’t do that when doing international moves and had to just suck it up.
We’ve had some brilliant packers who have not had a single breakage - including getting complex lego models from
the U.K. to US in one piece - and we’ve had some not so good ones who have had several breakages including irreplaceable cake stands which were my Granny’s. (Definitely cried about that move)
In theory you sit back and let them get on with it. Move anything you don’t want them to pack onto the beds - they are always the last thing to be dismantled so it’s easy to just say not to touch anything on them. Alternatively, move things into your car. Put brightly coloured notes (post-its but stuck on with tape so they don’t fall off) on anything not going at all - things like washing machines, curtains, light shades etc. I also put labels on all the doors, for instance Bedroom 1, Living Room, Play Room etc. It means they will label the boxes with the names of what YOU call the room, not what they have decided to call it! (We once had a move where we had a load of boxes labelled “kids room” and we had no idea where the stuff in them came from - could have been any of the DC’s bedrooms, the playroom, or the family room!)
Personally, I watch some things being packed to make sure they are doing it well (I will always watch cake stands being done now) and then if I’m happy I leave them to it. I will speak up if I’m not happy about anything. No need to be rude, but you’re paying a lot of money for the service so you’re entitled to get what you’re paying for.
I provide cans of juice and wrapped biscuits/cereal bars and then just let them get on with it.