Difference between Buy-in and Auction

#1
Are they two sides of the same coin or there are more differences than eyes can meet? Can anybody spare a min to explain me the difference b/w above two, if there exist any?

I need to know this in the context of BSE's proposed changes in the Auction (shortages) functionality proposed recently. Is it the same as the buy-in they used to have earlier?

Many thanks in advance.
 

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