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I saw a recent patent application publication (by Microsoft) where it looks like a a same or equivalent application was submitted to the U.S. patent office and the Chinese equivalent, on the same day.

I was wondering what might be the tactic or strategy here.

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It is a strategy called filing a PCT application. The one with the WO # (Not a US normal U.S. application) is the publication of a Patent Cooperation Treaty application. It is a process run by WIPO in Geneva. You can file in Geneva but applicants generally file a PCT international application through their own country's patent office. A PCT application is a "world" application but there is no such thing as a world patent. What makes a PCT application an international application is that its filing is has the effect of filing 147 applications in that many locations. At some later date the applicant choses which branches to let die and which to keep going with (paying money per location) . The filing date of the PCT application is the filing date of all 147 applications. Later, going into the national stage in China is technically not filing a new application, it is just changing the state of the China instance of the PCT application. The WO # application is not a "regular" U.S. national patent application. It was just filed through the USPTO acting as a Receiving Office for WIPO. On the other hand it, of course, is technically a U.S. application just like it is technically a China application, and a Luxembourg and an Indonesia application (and the other 142 places).

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