Climbing is the extreme case
On a rope, without a partner there is no sport: it isn't that company is nicer, it's that you cannot belay yourself. It's the only one on this list where finding someone doesn't improve the plan, it makes it possible.
What gets agreed beforehand
- Bouldering and roped climbing aren't organised alike
- Bouldering, you can turn up and climb; on a rope you need someone to belay, and you need to trust them. They're two different meet-ups even at the same venue.
- The grade, and what kind
- A 6a slab and a 6a overhang are not the same day out. Naming the grade and the style avoids the afternoon where one climbs and the other watches.