This got me thinking about the difference between a call and a claim. The call signifies your intention to claim the tile, and the League has clarified that you must verbalize your call. The tile is claimed when you have either exposed or placed the called tile atop your rack. The League states in "Mah Jongg Made Easy," - the "call" or "take" has become the claimant's turn. (1984, 2018)
There's been some brouhaha going on about this being conducive to "aggressive" play. After all, if one is committed to taking the discard, it precludes the player next in turn from waking up and saying, "Oh, gee, I wanted that flower." The argument is that someone who is paying attention and ready to call will "slam" their tiles atop the rack in a display of sharklike rudeness. Poppycock. Why would this be more likely to happen than an alert player snatching the discard from the table and slamming it atop the rack?
Here is an analogy: In scenario A, a person orders printer ink from Amazon. She pays by credit card. It's delivered to her door the next day. In scenario B, she goes to Staples for her ink, which is kept behind the counter. She tells the cashier what she wants. The cashier hands her the ink and takes her money. In both scenarios, she owns the ink.
An exposure is the price you pay for a discard. Does it matter if you pay in advance (expose first and then take the discard) or take the discard first and then expose? I say no. In either case, the tile is claimed. The player next in turn is TOO LATE. (2013)