The following content is from Peter Gordon's post. I think the use case illustrated the use of the category system of Open Contacts quite well.
It is easier to think of it in terms of categories, than as keywords.
Have a play with the contact manager: Open Contacts.
You could fit a complete company structure into it.
I use it for other purposes. Lets say I went to school with
Fred and we are best mates. Since leaving school, he has become
my plumber and we belong to the same social tennis group.
Fred would be stored under People|Friends, People|Class Of 1990,
Trades|Plumber, People|Tennis. Now suppose Fred shifts house,
so his contact details change. As he is entered in a database,
changing the database entry once will be reflected in all the
links to Fred.
A search for Fred can be done on his name or more easily by
working down through the tree to a small subset which contains
his name. I use this feature frequently. If my washing machine
breaks I can't remember the name of the service people, but I can
quickly work through Home Maintenance|White Goods|Repairs to find
the info I require.