Double XP is awarded to players on specific dates announced by DICE. They may be available for everyone or exclusive to those who are Battlefield Premium members.
In Battlefield 3, Double XP only doubles the player's overall rank progression, not class or vehicle type progression.
In Battlefield 4, while Double XP is active, XP boosts can be used simultaneously with it, but are not themselves doubled. (i.e., Double XP (100% boost) + 50% Boost = 150% total boost)
A new feature in Battlefield 4, XP Boosts are obtained randomly through Battlepacks. Once obtained, a boost can be activated in the pause menu and will stay active for the next hour of gameplay in a multiplayer match. There are currently four types:
25% Boost - Awards 1.25 times normal XP
50% Boost - Awards 1.5 times normal XP
100% Boost - Awards 2 times normal XP
200% Boost - Awards 3 times normal XP
All available XP Boosts can be viewed in a "sum-up" in the right bottom corner of the menu (during a MP-match).
An XP boost may be replaced by another though all remaining time of the former will be lost.
A glitch may cause all XP Boosts to temporarily be unavailable for use.