Cover arcs are stories that appear on the "cover" of each manga chapter of One Piece. These arcs tell stories of characters that the Straw Hats have met, usually telling the story of what happened afterwards. While these arcs did not usually feature the Straw Hats, that has changed as of chapter 543. Since this chapter, all of the cover arcs have been two-part cover arcs that have been used to tell the different adventures the different Straw Hats are having (except Luffy) since they were sent to different islands. The advantage to using cover arcs rather than normal manga chapters is that they take up space that would normally be used for just a picture and have recently been used to advance the story of the different straw hats. All of the cover arcs are canon and some have direct impacts on the Straw Hats. Examples:
1. Buggy teaming up with Alvida. (they meet up with the Straw Hats at Loguetown)
2. Koby and Helmeppo joining Garp's division. (they meet up with the Straw Hats after Enis Lobby)
3. Django joining the Marines. (he meets them again when they try to escape from Arabasta)
4. Hachan meeting and befriending Camie. (they meet up with the Straw Hats in the Saobody Arc)
5. Ace searching and finding Blackbeard. (2 chapters are devoted to thier meeting and the subsiquent fight; Blackbeard wins and Ace is sent to Impel Down, is currently being sent to be excecuted, and Luffy is trying to rescue him)
6. The Baroque Works members Crocodile, Mr. 1, Bon Clay, and Mr. 3 are sent to Impel Down. (they have currently joined Luffy in his attempt to free Ace and all but Mr. 2 have escaped from Impel Down)
7. The multiple adventures of the Straw Hats.
Of the ten cover arcs, only four have not had a direct impact on the Straw Hat's adventure.
1. Wapol becoming CEO of a toy company using his powers.
2. Gedatsu accedently coming to Arabasta and starting a life there.
3. Eneru going to the moon.
4. CP9 and what happened after Enis Lobby.
It is possible that these might be incorperated into the main story at a later date.