Madbull Shark Hop Up Rubber 60 Degree

The Madbull Shark Hop Up Rubber 60 Degree is suitable for any AEG and there are two bucking sin the pack along with 2 nubs.

The number of degrees says something about the hardness of the rubber. 50 Degrees is the softest variant and 80 degrees the hardest variant. The different hardnesses can be applied in different ways in your AEG.
It makes no sense to use the hardest bucking when it's around freezing point because the cold makes a bucking stiffer and thus harder to adjust. Especially in cold temperatures, you want a softer bucking.

Also, the hardness of the bucking is also linked to the weight of the bb you are playing with. You can easily use a 0.30gr bb on a 50 degree bucking, but this will probably result in it needing to be replaced faster than when you use this weight on, say, a 70 degree bucking.

Ultimately, then, it comes down to personal preference.