Question: More explanation about custom local key at 5th and 6th

[tintin10q]

In the docs at https://vuex-orm.org/guide/model/relationships.html#many-to-many it says: "You may also define custom local key at 5th and 6th argument."

It then shows an example. However, it is not clear for me what the purpose of this is. Could someone explain to me what you would do with this local key or what the idea is. I think the example in the docs does not make it clear it just says that something is possible but I don't know what it is.

Thanks!