Which Locks Count Toward Lock Escalation?

A little while back I wrote about Why Indexes Reduce Locks for Update and Delete Queries.

I got a great question on the post from Zac:

What’s not super clear is why it takes out a lock on the whole table, is this because it does a lock escalation as a result of the Full Scan? Will this always happen, or is there a threshold of record update counts where this will occur?

This was tough to answer in just a comment, so I promised a full post on the topic.

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