Hello!

The "Mass Action" option involves changing the "settings" of horses rather than entering them into shows or training.

Mass Action options allow you to edit multiple horses at a time instead of painstakingly editting a single horse one after another. For example, if you decided you wanted all of your horses to suddenly be Barrel Racers but they all had different disciplines, the Mass Action allows you to change this setting on all of your horses at once. You can also do this with Careers. There are quite a few other options, such as putting up multiple horses for sale or stud/brood, as well as branding them, or even placing them in an auction.

The Danger Zone of the Mass Action involves three options that would remove any selected horses from your account. For example, one of the options is to Retire, meaning that any and all horses selected for this option would be removed from your account as well as the game, which means you can no longer care for, breed, or show those horses.

Hopefully this helped! If you need further assistance, please let me know! :)

Mass Actions are accessed via Horses > Manage. It allows you to do things like put multiple horses up for sale at once, or retire many at one time, etc.

Enter All and Train All are additional purchases that can be made through Market > Ingot Store > Account.

Oh, well that's disappointing. Perhaps putting an explanation of what it actually does on the upgrade menu would be a good idea. I see now that if I want to mass train/show horses I will have to spend even more money to buy those upgrades..