What does the "under additional review" mean?

I submit a service request sometime ago. Today I got a letter from USCIS saying something like "... we are active processing your case. However, it's under additional review. If you don't hear from us within 6 months, you can call back.

What does the "additional review" mean? Is it a bad thing? I don't see any soft LUD for my case even I got the letter.