3-4 point high approach low extension
This is a Kitani joseki from the 1940s - though not exclusive to him. Black aims to make a very solid stone.
Up to is standard. In this case leaves no weakness behind (compare with the case of at a). This is a tight way to play.
In this crosscut variation, White gets a small corner in sente, while Black, after at a is developing on both sides. Before AI this was considered favorable for Black. AI however likes White better here, with its preference for territory and sente.
Before AI, this was considered correct and has been played in 2001 in a number of pro games. Black takes a larger corner but White takes influence. AI thinks having this large corner territory in sente is very good for Black.
More variations at 3-4 point high approach low extension, cross-cut