Of these, the one-space extension on the third line is the least common. Since the two-space extension will be preferred for its efficiency in most cases, the one-space extension, made when the room for a two-space extension is available, could be is a sign of some deeper theoretical or tactical reasoning.
One place it does occur is in relation with the large low enclosure, which is insecure in the corner. When is played as a checking extension against , Black is thinking hard about the aji in the left corner.
I started a rec.games.go thread a little while ago based on this sort of example:
A tactical example that impressed me.
Simple? Nothing is really simple when your opponent plays the 5-5 point. This is Matthew Macfadyen 6d - Zhang Shutai 7d from game 4 of the 1995 British Championship. The choice of is presumably all to do with preserving the chance of White at a against Black at b. Don't think I'd have found this one.
JF How do we know who's Black and White? Is there an SL convention? FWIW I always put W first.
PS There is a Japanese name for the short extension on the side: Honinbo's ikken tobi.
Charles I always put Black first.