What is this corporate action (merger_update, merger_completion) mean?

ca_type = merger has two ca_sub_type ( merger_update and merger_completion). What these two mean?
API reference :

@kou A “Corporate Action” is a broad term which means any action a corporation makes which directly causes a material change to the company’s securities. In the above case the corporate action is a merger. The company is expecting to merge with another company. This would typically result in new stock being issued. There are many, often creative, corporate actions each with their own specific details making handling them automatically difficult.

Alpaca will take care of handing all account impacts a corporate action may have such as the addition or subtraction of securities. A corporate action notice is generally just for information only.