This enclosure is common in pro play, to develop the 3-3 point (preventing the 3-3 point shoulderhit). If the 5-4 point is played first, it looks unambitious to take the 3-3 point rather than make the small high shimari; and that development is rare in pro games.
White here is the normal corner probe. Black is the joseki answer most used in practice, but a and b are also seen. Note that this position also occurs in a typical 5-4 point tenuki joseki, by transposition.
Black in answer is rare, but has been played by Ishida Yoshio (1982-11-25 against Iwata Tatsuaki). In that game Black had the marked stone in place. In general the exchange - looks like Black being forced (kikasare). This formation occurs also with the 3445 enclosure.
In contrast here would only be played as a ko threat. Even then it is rare (one example in the Kajiwara-Kitani game in The Direction of Play). If Black answered at a that would be a local loss for White (i.e. the threat could be considered loss-making); in particular because the chance to play the probe discussed above is now gone.
Bill Spight suggests Black at b as alternate answer (since Black at a feels like kikasare).
It seems, though, that between strong players will only be played when it is a ko threat that Black can't afford to answer; when White will cut through.
From a game Rui Naiwei-Hua Xueming in 1978. Black played to secure the corner (rather than a) because White already occupied the 9-3 and 3-9 points.