Drop chance is the official term used exclusively in Asphalt Xtreme as an equivalent to "drop rate". Drop chances are listed in tables that can be reached by tapping on a box in the shop and then on the purple box info icon next to the box name.
Terminology
Drop chance
There are two important differences to the term "drop rate":
 In Asphalt Xtreme, "drop" means that a random number of the same card is granted. Mathematically, a drop consists of two nested, but different random processes: The first determines the card type, the second determines the number of cards. For example, if a Box grants 4 drops, then players will get random amounts of 4 different (or in rare cases, equal) card types. The real number of cards received from 4 drops can vary from 4 to 40, assumed that the maximum number of cards per drop is 10. Some boxes can grant even higher amounts of cards per drop.
 Contrary to other Asphalt games, the use of the word "chance" implies that "drop chance" not only denotes the average frequency of a card in the long run (= its drop rate, an expected value), but also explicitly means the probability of getting it. This is possible because Asphalt Xtreme boxes do not have guarantees for certain cards. If a box contained a guaranteed card, the probability of getting it would always be 100 %, whereas the expected value would be different because the card does not make up 100 % of the box content.
According to the official tables, the drop chances of limitedtime boxes (like special offers and event boxes) are all the same. It is striking that in this case, the term "approximate drop rates" is used instead of "drop chances". It would have to be analyzed statistically if this means that these boxes do have differences and the figures are just average values.
Furthermore it would have to be examined if "drop chance" refers to the abovementioned "drops" or to the total number of received cards. For example, let a simplified box grant only ClassD Tools or ClassC Tools, both with a drop chance of 50 %.
 If the "drop chance" percentage referred to "drops", this would only mean that in the long run, there would be a balance between ClassC and ClassD drops, but the actual number of ClassC cards could vary from 9.09 % or 90.91 % of all cards—which is such a wide span that it makes strategic planning impossible and renders "drop chance" information misleading and useless.
 The only way the drop chance value could provide fair und useful information would be if it referred to the final number of cards and not to the number of times a random amount of the card is dropped. A drop chance of 15 % for a card type would mean that players could be sure that in the long run, 15 % of all received cards would be of this type. This is guaranteed by the law of large numbers.
Power level
The power level is a classification of basic boxes and can take values from 1 to 4. The higher the power level, the "better" the drop chances.In reality, the power level is not a box property but simply a measure of how many boxes a player has opened. The progress is displayed by a bar when the player taps on a box in the shop. Once the bar is full, the player advances to the next level. Opening 1Card Boxes does not count to the player's power level progress. The power level has no influence on the drop chances of limitedtime boxes; drop chances are the same for all levels.
It is questionable if a higher power level generally means "better" rewards. For example, a level 1 Iron Box drops Blueprints of all classes, whereas level 2 and 3 Iron Boxes only drop ClassD and C Blueprints. Even a level 4 Iron Box does not grant ClassS Blueprints. This is somewhat hidden in the confusing structure of the official drop chance tables, but can be clearly seen in the tables below.
Power value
The power value of a basic box is a number intended to indicate the quality of its drops. It increases with the player's power level. As with the term "power level", the value seems arbitrary and of no use for the player. Limitedtime boxes have no power value. It is unclear if 1Card Boxes have a power value as there is no way to display them on a separate screen like basic boxes.
Official values
Basic boxes 
Limitedtime boxes  

