I recently saw that Musnad Ahmad contains 70% sunni hadith, what does it mean?
What is a sunni hadith? What is its specialty?
As @sassir said in his comment all Muslim sects have their own sources (they only have a consensus about the qur'an itself).
So when I say sunni hadith I mean hadith compilations which are wide spread and known or used by sunni scholars and references starting with the two Sahih books of imam al-Bukhari and Muslim. See for example Are there commentaries of Hadith collections outside the 7 most known books? and Is Musnad Ahmed a book of Sahih Ahaadith? where some major works are mentioned.
On the other hand shi'a hadith sources are mentioned in
How many hadiths are in کتب الاربعه ? (Main Shia Books).
Maybe also relevant is What theological and historical factors distinguish Ibadi Islam from either Shia or Sunni schools?