Contact play, diagonal away
This is not joseki: Black was very strong on the upper side and wanted sente to play at a and complete his formation.
This idea of the 'diagonal away' is unusual.
On general principles of shape, one expects Black at answer at a, b or c (the play at the marked point being bad shape).
Exactly this shape does occur in a number of tenuki joseki; for example
- 3-4 point high approach two-space high pincer, ogeima, tenuki
- 4-5 point 4-3 approach ogeima, tenuki.