Depends on a classification you are going to use.
There are for example nodes that are configured (at build time) for so called FULL NODE or, the other type: LOW MEMORY NODE.
For a consensus all we need is LOW MEMORY NODE.
Seed nodes and Witness node are both LOW MEMORY NODEs.
In fact, RPC nodes doesn't always need to be configured as FULL NODE. It all depends on what RPC calls it's gonig to support.
I think that's a good subject for one of Steem Pressure episodes :-)
I've looked in to witness and seed nodes, but it looks like a lot of the information might be outdated a little bit now (most of the info was dating 1-2 years back). So an updated post would be great
Next Steem Pressure episode will be about setting up a seed node.