Slicing is only available/required on repeating elements. After a repeating element has been initially constrained, you can slice the element by clicking the scissors icon in the top-right like so:

This will create a new slice for the repeating element, and auto-generate the slice name. You can change the slice name to be more descriptive of what the slice represents. After creating a slice of the repeating element, you should go back to the top-level repeating element and specify discriminators for it. A new tab will appear when you have selected the the top-level repeating element that indicates “Slicing”, which allows you to specify details such as the “discriminator”.