What Twitter's application to trademark 'subtweet' meansDec 5(AZINS) Filed on 30 October 2015, Twitter's application to trademark "subtweet" was opened for opposition on 19 November, however as if no one opposes the application, "subtweet" will soon be a trademark of Twitter. The application was filed soon after Jack Dorsey was made permanent CEO.

Owning a trademark does not mean Twitter will own the word but "subtweet" will stand for its brand identification in a way.

According to gizmodo's report Twitter could then use it exclusively for commercial settings meaning that no new product could be named as and sold with "subtweet" on them unless it is by Twitter. Which means using the word "subtweet" on shirts or stationary and so on could fall under this category.
The report speculates the reason as "The reason people think that trademarking something is a big deal is that not enforcing it can lead to the mark growing weak, making it harder to enforce in courts."

Here is a copy of the application as tweeted by Alex Werpins handle,
 
A statement to the Verge by Twitter also bolsters the above speculation, “When you need to protect your namespace from people who would misuse it. #keepingit.”
Another reason why Twitter could be putting it all on getting "subtweet" trademarked is because it had failed to trademark “tweet” while applications continued to make use of it.

While Twitter learned it the hard way with its battle for trademarking "tweet" it hopes to see how far it can go with its application for "subtweet".