4473 enclosure 3-3 invasion, block on inner side
When Black plays at here in reply to the 3-3 invasion , there will be some important reason to do with strategy.
If were closer, the play at would be weak. As it is, it is somewhat overconcentrated. Black may intend to recapture the corner, as in the main line below. This may well depend on a ladder relationship.
This idea occurs quite often in pro games, from Shusai's time to today.
This is the most common continuation. Black recaptures the corner, apparently in gote.
Note that Black may only want to play this when at can be answered by capturing in a ladder.
After these plays, White at the circled point is very big: it can create shortage of liberties for Black after White at a.