Madbull 60 Degree Normal Hopup Bucking

The Madbull 60 Degree Normal Hopup Bucking is a relatively soft bucking and there are 2 buckings in the pack including 2 nubs.

The number of degrees says something about the hardness of the rubber. 50 Degrees is the softest version and 80 degrees the hardest. The different hardnesses can be used 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 sooner than if you use this weight on, say, a 70-degree bucking. Also, the higher degree buckings are more suitable for airsoft replicas with higher Joule values like a DMR.

Ultimately, then, it comes down to personal preference.