This might seem like a minor point. Sir Ranulph Fiennes was explaining that climbing Mount Everest was not like going for a walk in the park. If someone was giving you directions around town, they would say something like 'turn right at the Red Fox', the 'Red Fox' being the name of a pub.
In Mount Everest it slightly different, there were very few pubs but quite a lot of dead bodies. He gave the example:
Turn right at the green boots
Normally, you would expect someone to say:
Turn right by the green boots.
I am not sure how this relates to 'word types', whether a 'pub' is a different word type to 'green boots'. But I feel that my second example would be a more typical expression.
He just happened to say 'at the green boots' because he had just been talking about the 'Red Fox'. I am sure that makes sense!
Perhaps it is the difference between 'nouns' and 'Proper nouns'?
1. Turn right at the X
2. Turn right by the X
I would say that "at" in #1 pinpoints the location of the boots, while "by" in #2 gives the location of the "turn".
MrP