From Halopedia, the Halo wiki
Extra Credit is an achievement in Halo: Spartan Assault. It is unlocked by completing 75 mission specific Assault Ops in the Windows version of the game, and 90 in the Xbox 360 and Xbox One versions. The achievement is worth 10 Gamerscore on Windows, 15 Gamerscore on Xbox 360, and 30 Gamerscore on Xbox One. It is represented by a circle with an arrow pointing to the right.
When first released on Windows, Spartan Assault featured three mission specific Assault Ops for each of the game's 25 missions. In order to unlock the achievement, players had to complete every single challenge. With the release of the Operation F: Hydra title update on August 29, 2013, fifteen more were added, bringing the total to 90. Completing the 15 mission specific challenges in Operation F will unlock the A for Effort achievement, worth 5 Gamerscore, and allow the player to forgo 15 of the more difficult original challenges.
The Xbox One and Xbox 360 versions of Spartan Assault - released December 23, 2013 and January 31, 2014, respectively - do not include the A for Effort achievement. Instead, players must complete all 90 Assault Ops, to unlock the Extra Credit achievement. The Gamerscore from A for Effort is added to Extra Credit, bringing the totals to 15 and 30 Gamerscore on Xbox 360 and Xbox One respectively.
The challenges carry across all Microsoft platforms (but not Steam or iOS). Any progress or completion on one Microsoft platform contributes toward this achievement on all others.
If a player has already unlocked this achievement on one Windows or Xbox platform, it will immediately unlock the first time they sign in on another Windows or Xbox platform. If the player unlocked the achievement first on a Windows 8 or Windows Phone 8 device by completing 75 challenges, they will still need to complete the remaining 15 to unlock it on Xbox 360 and Xbox One.